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A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off 1 a separate process -- or because a particular builtin needs direct access to the shell internals.
When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called forking. This new process is the child, and the process that forked it off is the parent. While the child process is doing its work, the parent process is still executing.
Note that while a parent process gets the process ID of the child process, and can thus pass arguments to it, the reverse is not true. This can create problems that are subtle and hard to track down.
Example 15-1. A script that spawns multiple instances of itself
#!/bin/bash
# spawn.sh
PIDS=$(pidof sh $0) # Process IDs of the various instances of this script.
P_array=( $PIDS ) # Put them in an array (why?).
echo $PIDS # Show process IDs of parent and child processes.
let "instances = ${#P_array[*]} - 1" # Count elements, less 1.
# Why subtract 1?
echo "$instances instance(s) of this script running."
echo "[Hit Ctl-C to exit.]"; echo
sleep 1 # Wait.
sh $0 # Play it again, Sam.
exit 0 # Not necessary; script will never get to here.
# Why not?
# After exiting with a Ctl-C,
#+ do all the spawned instances of the script die?
# If so, why?
# Note:
# ----
# Be careful not to run this script too long.
# It will eventually eat up too many system resources.
# Is having a script spawn multiple instances of itself
#+ an advisable scripting technique.
# Why or why not?
Generally, a Bash builtin does not fork a subprocess when it executes within a script. An external system command or filter in a script usually will fork a subprocess.
A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For example, the Bash echo command is not the same as /bin/echo, although their behavior is almost identical.
#!/bin/bash echo "This line uses the \"echo\" builtin." /bin/echo "This line uses the /bin/echo system command."
A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a subunit of a command construct.2
prints (to stdout) an expression or variable (see Example 4-1).
echo Hello echo $a
An echo requires the -e option to print escaped characters. See Example 5-2.
Normally, each echo command prints a terminal newline, but the -n option suppresses this.
An
echo can be used to feed a sequence of commands down a pipe.
if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] then echo "$VAR contains the substring sequence \"txt\"" fi
An
echo, in combination with command substitution can set a variable.
a=`echo "HELLO" | tr A-Z a-z`
See also Example 16-22, Example 16-3, Example 16-47, and Example 16-48.
Be aware that echo `command` deletes any linefeeds that the output of command generates.
The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to echo. Then echo outputs these arguments, separated by spaces.
bash$ ls -l /usr/share/apps/kjezz/sounds
-rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au
-rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au
bash$ echo `ls -l /usr/share/apps/kjezz/sounds`
total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root ...
So, how can we embed a linefeed within an echoed character string?
# Embedding a linefeed?
echo "Why doesn't this string \n split on two lines?"
# Doesn't split.
# Let's try something else.
echo
echo $"A line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# But, is the "$" variable prefix really necessary?
echo
echo "This string splits
on two lines."
# No, the "$" is not needed.
echo
echo "---------------"
echo
echo -n $"Another line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# Even the -n option fails to suppress the linefeed here.
echo
echo
echo "---------------"
echo
echo
# However, the following doesn't work as expected.
# Why not? Hint: Assignment to a variable.
string1=$"Yet another line of text containing
a linefeed (maybe)."
echo $string1
# Yet another line of text containing a linefeed (maybe).
# ^
# Linefeed becomes a space.
# Thanks, Steve Parker, for pointing this out.
This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar.
bash$ type -a echo
echo is a shell builtin
echo is /bin/echo
The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language printf() library function, and its syntax is somewhat different.
printf format-string... parameter...
This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the printf manpage (of the system command) for in-depth coverage.
Older versions of Bash may not support
printf.
Example 15-2. printf in action
#!/bin/bash
# printf demo
declare -r PI=3.14159265358979 # Read-only variable, i.e., a constant.
declare -r DecimalConstant=31373
Message1="Greetings,"
Message2="Earthling."
echo
printf "Pi to 2 decimal places = %1.2f" $PI
echo
printf "Pi to 9 decimal places = %1.9f" $PI # It even rounds off correctly.
printf "\n" # Prints a line feed,
# Equivalent to 'echo' . . .
printf "Constant = \t%d\n" $DecimalConstant # Inserts tab (\t).
printf "%s %s \n" $Message1 $Message2
echo
# ==========================================#
# Simulation of C function, sprintf().
# Loading a variable with a formatted string.
echo
Pi12=$(printf "%1.12f" $PI)
echo "Pi to 12 decimal places = $Pi12" # Roundoff error!
Msg=`printf "%s %s \n" $Message1 $Message2`
echo $Msg; echo $Msg
# As it happens, the 'sprintf' function can now be accessed
#+ as a loadable module to Bash,
#+ but this is not portable.
exit 0
Formatting error messages is a useful application of printf
E_BADDIR=85
var=nonexistent_directory
error()
{
printf "$@" >&2
# Formats positional params passed, and sends them to stderr.
echo
exit $E_BADDIR
}
cd $var || error $"Can't cd to %s." "$var"
# Thanks, S.C.
See also Example 36-15.
"Reads" the value of a variable from stdin, that is, interactively fetches input from the keyboard. The -a option lets read get array variables (see Example 27-6).
Example 15-3. Variable assignment, using read
#!/bin/bash # "Reading" variables. echo -n "Enter the value of variable 'var1': " # The -n option to echo suppresses newline. read var1 # Note no '$' in front of var1, since it is being set. echo "var1 = $var1" echo # A single 'read' statement can set multiple variables. echo -n "Enter the values of variables 'var2' and 'var3' " echo =n "(separated by a space or tab): " read var2 var3 echo "var2 = $var2 var3 = $var3" # If you input only one value, #+ the other variable(s) will remain unset (null). exit 0
A read without an associated variable assigns its input to the dedicated variable $REPLY.
Example 15-4. What happens when read has no variable
#!/bin/bash
# read-novar.sh
echo
# -------------------------- #
echo -n "Enter a value: "
read var
echo "\"var\" = "$var""
# Everything as expected here.
# -------------------------- #
echo
# ------------------------------------------------------------------- #
echo -n "Enter another value: "
read # No variable supplied for 'read', therefore...
#+ Input to 'read' assigned to default variable, $REPLY.
var="$REPLY"
echo "\"var\" = "$var""
# This is equivalent to the first code block.
# ------------------------------------------------------------------- #
echo
echo "========================="
echo
# This example is similar to the "reply.sh" script.
# However, this one shows that $REPLY is available
#+ even after a 'read' to a variable in the conventional way.
# ================================================================= #
# In some instances, you might wish to discard the first value read.
# In such cases, simply ignore the $REPLY variable.
{ # Code block.
read # Line 1, to be discarded.
read line2 # Line 2, saved in variable.
} <$0
echo "Line 2 of this script is:"
echo "$line2" # # read-novar.sh
echo # #!/bin/bash line discarded.
# See also the soundcard-on.sh script.
exit 0
Normally, inputting a \ suppresses a newline during input to a read. The -r option causes an inputted \ to be interpreted literally.
Example 15-5. Multi-line input to read
#!/bin/bash
echo
echo "Enter a string terminated by a \\, then press <ENTER>."
echo "Then, enter a second string (no \\ this time), and again press <ENTER>."
read var1 # The "\" suppresses the newline, when reading $var1.
# first line \
# second line
echo "var1 = $var1"
# var1 = first line second line
# For each line terminated by a "\"
#+ you get a prompt on the next line to continue feeding characters into var1.
echo; echo
echo "Enter another string terminated by a \\ , then press <ENTER>."
read -r var2 # The -r option causes the "\" to be read literally.
# first line \
echo "var2 = $var2"
# var2 = first line \
# Data entry terminates with the first <ENTER>.
echo
exit 0
The read command has some interesting options that permit echoing a prompt and even reading keystrokes without hitting ENTER.
# Read a keypress without hitting ENTER. read -s -n1 -p "Hit a key " keypress echo; echo "Keypress was "\"$keypress\""." # -s option means do not echo input. # -n N option means accept only N characters of input. # -p option means echo the following prompt before reading input. # Using these options is tricky, since they need to be in the correct order.
The -n option to read also allows detection of the arrow keys and certain of the other unusual keys.
Example 15-6. Detecting the arrow keys
#!/bin/bash
# arrow-detect.sh: Detects the arrow keys, and a few more.
# Thank you, Sandro Magi, for showing me how.
# --------------------------------------------
# Character codes generated by the keypresses.
arrowup='\[A'
arrowdown='\[B'
arrowrt='\[C'
arrowleft='\[D'
insert='\[2'
delete='\[3'
# --------------------------------------------
SUCCESS=0
OTHER=65
echo -n "Press a key... "
# May need to also press ENTER if a key not listed above pressed.
read -n3 key # Read 3 characters.
echo -n "$key" | grep "$arrowup" #Check if character code detected.
if [ "$?" -eq $SUCCESS ]
then
echo "Up-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowdown"
if [ "$?" -eq $SUCCESS ]
then
echo "Down-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowrt"
if [ "$?" -eq $SUCCESS ]
then
echo "Right-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowleft"
if [ "$?" -eq $SUCCESS ]
then
echo "Left-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$insert"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Insert\" key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$delete"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Delete\" key pressed."
exit $SUCCESS
fi
echo " Some other key pressed."
exit $OTHER
# ========================================= #
# Mark Alexander came up with a simplified
#+ version of the above script (Thank you!).
# It eliminates the need for grep.
#!/bin/bash
uparrow=$'\x1b[A'
downarrow=$'\x1b[B'
leftarrow=$'\x1b[D'
rightarrow=$'\x1b[C'
read -s -n3 -p "Hit an arrow key: " x
case "$x" in
$uparrow)
echo "You pressed up-arrow"
;;
$downarrow)
echo "You pressed down-arrow"
;;
$leftarrow)
echo "You pressed left-arrow"
;;
$rightarrow)
echo "You pressed right-arrow"
;;
esac
exit $?
# ========================================= #
# Antonio Macchi has a simpler alternative.
#!/bin/bash
while true
do
read -sn1 a
test "$a" == `echo -en "\e"` || continue
read -sn1 a
test "$a" == "[" || continue
read -sn1 a
case "$a" in
A) echo "up";;
B) echo "down";;
C) echo "right";;
D) echo "left";;
esac
done
# ========================================= #
# Exercise:
# --------
# 1) Add detection of the "Home," "End," "PgUp," and "PgDn" keys.
The -n option to
read will not detect the ENTER (newline) key.
The -t option to read permits timed input (see Example 9-4 and Example A-41).
The -u option takes the file descriptor of the target file.
The read command may also "read" its variable value from a file redirected to stdin. If the file contains more than one line, only the first line is assigned to the variable. If read has more than one parameter, then each of these variables gets assigned a successive whitespace-delineated string. Caution!
Example 15-7. Using read with file redirection
#!/bin/bash read var1 <data-file echo "var1 = $var1" # var1 set to the entire first line of the input file "data-file" read var2 var3 <data-file echo "var2 = $var2 var3 = $var3" # Note non-intuitive behavior of "read" here. # 1) Rewinds back to the beginning of input file. # 2) Each variable is now set to a corresponding string, # separated by whitespace, rather than to an entire line of text. # 3) The final variable gets the remainder of the line. # 4) If there are more variables to be set than whitespace-terminated strings # on the first line of the file, then the excess variables remain empty. echo "------------------------------------------------" # How to resolve the above problem with a loop: while read line do echo "$line" done <data-file # Thanks, Heiner Steven for pointing this out. echo "------------------------------------------------" # Use $IFS (Internal Field Separator variable) to split a line of input to # "read", if you do not want the default to be whitespace. echo "List of all users:" OIFS=$IFS; IFS=: # /etc/passwd uses ":" for field separator. while read name passwd uid gid fullname ignore do echo "$name ($fullname)" done </etc/passwd # I/O redirection. IFS=$OIFS # Restore original $IFS. # This code snippet also by Heiner Steven. # Setting the $IFS variable within the loop itself #+ eliminates the need for storing the original $IFS #+ in a temporary variable. # Thanks, Dim Segebart, for pointing this out. echo "------------------------------------------------" echo "List of all users:" while IFS=: read name passwd uid gid fullname ignore do echo "$name ($fullname)" done </etc/passwd # I/O redirection. echo echo "\$IFS still $IFS" exit 0
Piping output to a read, using echo to set variables will fail.
Yet, piping the output of cat seems to work.
cat file1 file2 | while read line do echo $line done
However, as Bjön Eriksson shows:
Example 15-8. Problems reading from a pipe
#!/bin/sh
# readpipe.sh
# This example contributed by Bjon Eriksson.
### shopt -s lastpipe
last="(null)"
cat $0 |
while read line
do
echo "{$line}"
last=$line
done
echo
echo "++++++++++++++++++++++"
printf "\nAll done, last: $last\n" # The output of this line
#+ changes if you uncomment line 5.
# (Bash, version -ge 4.2 required.)
exit 0 # End of code.
# (Partial) output of script follows.
# The 'echo' supplies extra brackets.
#############################################
./readpipe.sh
{#!/bin/sh}
{last="(null)"}
{cat $0 |}
{while read line}
{do}
{echo "{$line}"}
{last=$line}
{done}
{printf "nAll done, last: $lastn"}
All done, last: (null)
The variable (last) is set within the loop/subshell
but its value does not persist outside the loop.
The gendiff script, usually found in /usr/bin on many Linux distros, pipes the output of find to a while read construct.
find $1 \( -name "*$2" -o -name ".*$2" \) -print | while read f; do . . .
It is possible to paste text into the input field of a read (but not multiple lines!). See Example A-38.
The familiar cd change directory command finds use in scripts where execution of a command requires being in a specified directory.
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
[from the previously cited example by Alan Cox]
The -P (physical) option to cd causes it to ignore symbolic links.
cd - changes to $OLDPWD, the previous working directory.
The
cd command does not function as expected when presented with two forward slashes.
bash$ cd //
bash$ pwd
//
The output should, of course, be /. This is a problem both from the command-line and in a script.
Print Working Directory. This gives the user's (or script's) current directory (see Example 15-9). The effect is identical to reading the value of the builtin variable $PWD.
This command set is a mechanism for bookmarking working directories, a means of moving back and forth through directories in an orderly manner. A pushdown stack is used to keep track of directory names. Options allow various manipulations of the directory stack.
pushd dir-name pushes the path dir-name onto the directory stack and simultaneously changes the current working directory to dir-name
popd removes (pops) the top directory path name off the directory stack and simultaneously changes the current working directory to that directory popped from the stack.
dirs lists the contents of the directory stack (compare this with the $DIRSTACK variable). A successful pushd or popd will automatically invoke dirs.
Scripts that require various changes to the current working directory without hard-coding the directory name changes can make good use of these commands. Note that the implicit $DIRSTACK array variable, accessible from within a script, holds the contents of the directory stack.
Example 15-9. Changing the current working directory
#!/bin/bash dir1=/usr/local dir2=/var/spool pushd $dir1 # Will do an automatic 'dirs' (list directory stack to stdout). echo "Now in directory `pwd`." # Uses back-quoted 'pwd'. # Now, do some stuff in directory 'dir1'. pushd $dir2 echo "Now in directory `pwd`." # Now, do some stuff in directory 'dir2'. echo "The top entry in the DIRSTACK array is $DIRSTACK." popd echo "Now back in directory `pwd`." # Now, do some more stuff in directory 'dir1'. popd echo "Now back in original working directory `pwd`." exit 0 # What happens if you don't 'popd' -- then exit the script? # Which directory do you end up in? Why?
The let command carries out arithmetic operations on variables. 3 In many cases, it functions as a less complex version of expr.
Example 15-10. Letting let do arithmetic.
#!/bin/bash
echo
let a=11 # Same as 'a=11'
let a=a+5 # Equivalent to let "a = a + 5"
# (Double quotes and spaces make it more readable.)
echo "11 + 5 = $a" # 16
let "a <<= 3" # Equivalent to let "a = a << 3"
echo "\"\$a\" (=16) left-shifted 3 places = $a"
# 128
let "a /= 4" # Equivalent to let "a = a / 4"
echo "128 / 4 = $a" # 32
let "a -= 5" # Equivalent to let "a = a - 5"
echo "32 - 5 = $a" # 27
let "a *= 10" # Equivalent to let "a = a * 10"
echo "27 * 10 = $a" # 270
let "a %= 8" # Equivalent to let "a = a % 8"
echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"
# 6
# Does "let" permit C-style operators?
# Yes, just as the (( ... )) double-parentheses construct does.
let a++ # C-style (post) increment.
echo "6++ = $a" # 6++ = 7
let a-- # C-style decrement.
echo "7-- = $a" # 7-- = 6
# Of course, ++a, etc., also allowed . . .
echo
# Trinary operator.
# Note that $a is 6, see above.
let "t = a<7?7:11" # True
echo $t # 7
let a++
let "t = a<7?7:11" # False
echo $t # 11
exit
The
let command can, in certain contexts, return a surprising exit status.
# Evgeniy Ivanov points out:
var=0
echo $? # 0
# As expected.
let var++
echo $? # 1
# The command was successful, so why isn't $?=0 ???
# Anomaly!
let var++
echo $? # 0
# As expected.
# Likewise . . .
let var=0
echo $? # 1
# The command was successful, so why isn't $?=0 ???
# However, as Jeff Gorak points out,
#+ this is part of the design spec for 'let' . . .
# "If the last ARG evaluates to 0, let returns 1;
# let returns 0 otherwise." ['help let']
eval arg1 [arg2] ... [argN]
Combines the arguments in an expression or list of expressions and evaluates them. Any variables within the expression are expanded. The net result is to convert a string into a command.
The
eval command can be used for code generation from the command-line or within a script.
bash$ command_string="ps ax"
bash$ process="ps ax"
bash$ eval "$command_string" | grep "$process"
26973 pts/3 R+ 0:00 grep --color ps ax
26974 pts/3 R+ 0:00 ps ax
Each invocation of eval forces a re-evaluation of its arguments.
a='$b'
b='$c'
c=d
echo $a # $b
# First level.
eval echo $a # $c
# Second level.
eval eval echo $a # d
# Third level.
# Thank you, E. Choroba.
Example 15-11. Showing the effect of eval
#!/bin/bash
# Exercising "eval" ...
y=`eval ls -l` # Similar to y=`ls -l`
echo $y #+ but linefeeds removed because "echoed" variable is unquoted.
echo
echo "$y" # Linefeeds preserved when variable is quoted.
echo; echo
y=`eval df` # Similar to y=`df`
echo $y #+ but linefeeds removed.
# When LF's not preserved, it may make it easier to parse output,
#+ using utilities such as "awk".
echo
echo "==========================================================="
echo
eval "`seq 3 | sed -e 's/.*/echo var&=ABCDEFGHIJ/'`"
# var1=ABCDEFGHIJ
# var2=ABCDEFGHIJ
# var3=ABCDEFGHIJ
echo
echo "==========================================================="
echo
# Now, showing how to do something useful with "eval" . . .
# (Thank you, E. Choroba!)
version=3.4 # Can we split the version into major and minor
#+ part in one command?
echo "version = $version"
eval major=${version/./;minor=} # Replaces '.' in version by ';minor='
# The substitution yields '3; minor=4'
#+ so eval does minor=4, major=3
echo Major: $major, minor: $minor # Major: 3, minor: 4
Example 15-12. Using eval to select among variables
#!/bin/bash
# arr-choice.sh
# Passing arguments to a function to select
#+ one particular variable out of a group.
arr0=( 10 11 12 13 14 15 )
arr1=( 20 21 22 23 24 25 )
arr2=( 30 31 32 33 34 35 )
# 0 1 2 3 4 5 Element number (zero-indexed)
choose_array ()
{
eval array_member=\${arr${array_number}[element_number]}
# ^ ^^^^^^^^^^^^
# Using eval to construct the name of a variable,
#+ in this particular case, an array name.
echo "Element $element_number of array $array_number is $array_member"
} # Function can be rewritten to take parameters.
array_number=0 # First array.
element_number=3
choose_array # 13
array_number=2 # Third array.
element_number=4
choose_array # 34
array_number=3 # Null array (arr3 not allocated).
element_number=4
choose_array # (null)
# Thank you, Antonio Macchi, for pointing this out.
Example 15-13. Echoing the command-line parameters
#!/bin/bash
# echo-params.sh
# Call this script with a few command-line parameters.
# For example:
# sh echo-params.sh first second third fourth fifth
params=$# # Number of command-line parameters.
param=1 # Start at first command-line param.
while [ "$param" -le "$params" ]
do
echo -n "Command-line parameter "
echo -n \$$param # Gives only the *name* of variable.
# ^^^ # $1, $2, $3, etc.
# Why?
# \$ escapes the first "$"
#+ so it echoes literally,
#+ and $param dereferences "$param" . . .
#+ . . . as expected.
echo -n " = "
eval echo \$$param # Gives the *value* of variable.
# ^^^^ ^^^ # The "eval" forces the *evaluation*
#+ of \$$
#+ as an indirect variable reference.
(( param ++ )) # On to the next.
done
exit $?
# =================================================
$ sh echo-params.sh first second third fourth fifth
Command-line parameter $1 = first
Command-line parameter $2 = second
Command-line parameter $3 = third
Command-line parameter $4 = fourth
Command-line parameter $5 = fifth
Example 15-14. Forcing a log-off
#!/bin/bash
# Killing ppp to force a log-off.
# For dialup connection, of course.
# Script should be run as root user.
SERPORT=ttyS3
# Depending on the hardware and even the kernel version,
#+ the modem port on your machine may be different --
#+ /dev/ttyS1 or /dev/ttyS2.
killppp="eval kill -9 `ps ax | awk '/ppp/ { print $1 }'`"
# -------- process ID of ppp -------
$killppp # This variable is now a command.
# The following operations must be done as root user.
chmod 666 /dev/$SERPORT # Restore r+w permissions, or else what?
# Since doing a SIGKILL on ppp changed the permissions on the serial port,
#+ we restore permissions to previous state.
rm /var/lock/LCK..$SERPORT # Remove the serial port lock file. Why?
exit $?
# Exercises:
# ---------
# 1) Have script check whether root user is invoking it.
# 2) Do a check on whether the process to be killed
#+ is actually running before attempting to kill it.
# 3) Write an alternate version of this script based on 'fuser':
#+ if [ fuser -s /dev/modem ]; then . . .
Example 15-15. A version of rot13
#!/bin/bash
# A version of "rot13" using 'eval'.
# Compare to "rot13.sh" example.
setvar_rot_13() # "rot13" scrambling
{
local varname=$1 varvalue=$2
eval $varname='$(echo "$varvalue" | tr a-z n-za-m)'
}
setvar_rot_13 var "foobar" # Run "foobar" through rot13.
echo $var # sbbone
setvar_rot_13 var "$var" # Run "sbbone" through rot13.
# Back to original variable.
echo $var # foobar
# This example by Stephane Chazelas.
# Modified by document author.
exit 0
Here is another example of using eval to evaluate a complex expression, this one from YongYe's Tetris game script.
eval ${1}+=\"${x} ${y} \"
Example A-53 uses eval to convert array elements into a command list.
The eval command occurs in the older version of indirect referencing.
eval var=\$$var
The eval command can be used to parameterize brace expansion.
The
eval command can be risky, and normally should be avoided when there exists a reasonable alternative. An eval $COMMANDS executes the contents of COMMANDS, which may contain such unpleasant surprises as rm -rf *. Running an eval on unfamiliar code written by persons unknown is living dangerously.
The set command changes the value of internal script variables/options. One use for this is to toggle option flags which help determine the behavior of the script. Another application for it is to reset the positional parameters that a script sees as the result of a command (set `command`). The script can then parse the fields of the command output.
Example 15-16. Using set with positional parameters
#!/bin/bash
# ex34.sh
# Script "set-test"
# Invoke this script with three command-line parameters,
# for example, "sh ex34.sh one two three".
echo
echo "Positional parameters before set \`uname -a\` :"
echo "Command-line argument #1 = $1"
echo "Command-line argument #2 = $2"
echo "Command-line argument #3 = $3"
set `uname -a` # Sets the positional parameters to the output
# of the command `uname -a`
echo
echo +++++
echo $_ # +++++
# Flags set in script.
echo $- # hB
# Anomalous behavior?
echo
echo "Positional parameters after set \`uname -a\` :"
# $1, $2, $3, etc. reinitialized to result of `uname -a`
echo "Field #1 of 'uname -a' = $1"
echo "Field #2 of 'uname -a' = $2"
echo "Field #3 of 'uname -a' = $3"
echo \#\#\#
echo $_ # ###
echo
exit 0
More fun with positional parameters.
Example 15-17. Reversing the positional parameters
#!/bin/bash
# revposparams.sh: Reverse positional parameters.
# Script by Dan Jacobson, with stylistic revisions by document author.
set a\ b c d\ e;
# ^ ^ Spaces escaped
# ^ ^ Spaces not escaped
OIFS=$IFS; IFS=:;
# ^ Saving old IFS and setting new one.
echo
until [ $# -eq 0 ]
do # Step through positional parameters.
echo "### k0 = "$k"" # Before
k=$1:$k; # Append each pos param to loop variable.
# ^
echo "### k = "$k"" # After
echo
shift;
done
set $k # Set new positional parameters.
echo -
echo $# # Count of positional parameters.
echo -
echo
for i # Omitting the "in list" sets the variable -- i --
#+ to the positional parameters.
do
echo $i # Display new positional parameters.
done
IFS=$OIFS # Restore IFS.
# Question:
# Is it necessary to set an new IFS, internal field separator,
#+ in order for this script to work properly?
# What happens if you don't? Try it.
# And, why use the new IFS -- a colon -- in line 17,
#+ to append to the loop variable?
# What is the purpose of this?
exit 0
$ ./revposparams.sh
### k0 =
### k = a b
### k0 = a b
### k = c a b
### k0 = c a b
### k = d e c a b
-
3
-
d e
c
a b
Invoking set without any options or arguments simply lists all the environmental and other variables that have been initialized.
bash$ set
AUTHORCOPY=/home/bozo/posts
BASH=/bin/bash
BASH_VERSION=$'2.05.8(1)-release'
...
XAUTHORITY=/home/bozo/.Xauthority
_=/etc/bashrc
variable22=abc
variable23=xzy
Using set with the -- option explicitly assigns the contents of a variable to the positional parameters. If no variable follows the -- it unsets the positional parameters.
Example 15-18. Reassigning the positional parameters
#!/bin/bash variable="one two three four five" set -- $variable # Sets positional parameters to the contents of "$variable". first_param=$1 second_param=$2 shift; shift # Shift past first two positional params. # shift 2 also works. remaining_params="$*" echo echo "first parameter = $first_param" # one echo "second parameter = $second_param" # two echo "remaining parameters = $remaining_params" # three four five echo; echo # Again. set -- $variable first_param=$1 second_param=$2 echo "first parameter = $first_param" # one echo "second parameter = $second_param" # two # ====================================================== set -- # Unsets positional parameters if no variable specified. first_param=$1 second_param=$2 echo "first parameter = $first_param" # (null value) echo "second parameter = $second_param" # (null value) exit 0
See also Example 11-2 and Example 16-56.
The unset command deletes a shell variable, effectively setting it to null. Note that this command does not affect positional parameters.
bash$ unset PATH
bash$ echo $PATH
bash$
Example 15-19. "Unsetting" a variable
#!/bin/bash
# unset.sh: Unsetting a variable.
variable=hello # Initialized.
echo "variable = $variable"
unset variable # Unset.
# In this particular context,
#+ same effect as: variable=
echo "(unset) variable = $variable" # $variable is null.
if [ -z "$variable" ] # Try a string-length test.
then
echo "\$variable has zero length."
fi
exit 0
In most contexts, an undeclared variable and one that has been unset are equivalent. However, the ${parameter:-default} parameter substitution construct can distinguish between the two.
The export 4 command makes available variables to all child processes of the running script or shell. One important use of the export command is in startup files, to initialize and make accessible environmental variables to subsequent user processes.
Unfortunately, there is no way to export variables back to the parent process, to the process that called or invoked the script or shell.
Example 15-20. Using export to pass a variable to an embedded awk script
#!/bin/bash
# Yet another version of the "column totaler" script (col-totaler.sh)
#+ that adds up a specified column (of numbers) in the target file.
# This uses the environment to pass a script variable to 'awk' . . .
#+ and places the awk script in a variable.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
export column_number
# Export column number to environment, so it's available for retrieval.
# -----------------------------------------------
awkscript='{ total += $ENVIRON["column_number"] }
END { print total }'
# Yes, a variable can hold an awk script.
# -----------------------------------------------
# Now, run the awk script.
awk "$awkscript" "$filename"
# Thanks, Stephane Chazelas.
exit 0
It is possible to initialize and export variables in the same operation, as in
export var1=xxx.
However, as Greg Keraunen points out, in certain situations this may have a different effect than setting a variable, then exporting it.
bash$ export var=(a b); echo ${var[0]}
(a b)
bash$ var=(a b); export var; echo ${var[0]}
a
A variable to be exported may require special treatment. See Example M-2.
The declare and typeset commands specify and/or restrict properties of variables.
Same as declare -r, sets a variable as read-only, or, in effect, as a constant. Attempts to change the variable fail with an error message. This is the shell analog of the C language const type qualifier.
This powerful tool parses command-line arguments passed to the script. This is the Bash analog of the getopt external command and the getopt library function familiar to C programmers. It permits passing and concatenating multiple options 5 and associated arguments to a script (for example scriptname -abc -e /usr/local).
The getopts construct uses two implicit variables. $OPTIND is the argument pointer (OPTion INDex) and $OPTARG (OPTion ARGument) the (optional) argument attached to an option. A colon following the option name in the declaration tags that option as having an associated argument.
A getopts construct usually comes packaged in a while loop, which processes the options and arguments one at a time, then increments the implicit $OPTIND variable to point to the next.
The arguments passed from the command-line to the script must be preceded by a dash (-). It is the prefixed - that lets getopts recognize command-line arguments as options. In fact, getopts will not process arguments without the prefixed -, and will terminate option processing at the first argument encountered lacking them.
getopts template differs slightly from the standard while loop, in that it lacks condition brackets.getopts construct is a highly functional replacement for the traditional getopt external command.
while getopts ":abcde:fg" Option
# Initial declaration.
# a, b, c, d, e, f, and g are the options (flags) expected.
# The : after option 'e' shows it will have an argument passed with it.
do
case $Option in
a ) # Do something with variable 'a'.
b ) # Do something with variable 'b'.
...
e) # Do something with 'e', and also with $OPTARG,
# which is the associated argument passed with option 'e'.
...
g ) # Do something with variable 'g'.
esac
done
shift $(($OPTIND - 1))
# Move argument pointer to next.
# All this is not nearly as complicated as it looks <grin>.
Example 15-21. Using getopts to read the options/arguments passed to a script
#!/bin/bash
# ex33.sh: Exercising getopts and OPTIND
# Script modified 10/09/03 at the suggestion of Bill Gradwohl.
# Here we observe how 'getopts' processes command-line arguments to script.
# The arguments are parsed as "options" (flags) and associated arguments.
# Try invoking this script with:
# 'scriptname -mn'
# 'scriptname -oq qOption' (qOption can be some arbitrary string.)
# 'scriptname -qXXX -r'
#
# 'scriptname -qr'
#+ - Unexpected result, takes "r" as the argument to option "q"
# 'scriptname -q -r'
#+ - Unexpected result, same as above
# 'scriptname -mnop -mnop' - Unexpected result
# (OPTIND is unreliable at stating where an option came from.)
#
# If an option expects an argument ("flag:"), then it will grab
#+ whatever is next on the command-line.
NO_ARGS=0
E_OPTERROR=85
if [ $# -eq "$NO_ARGS" ] # Script invoked with no command-line args?
then
echo "Usage: `basename $0` options (-mnopqrs)"
exit $E_OPTERROR # Exit and explain usage.
# Usage: scriptname -options
# Note: dash (-) necessary
fi
while getopts ":mnopq:rs" Option
do
case $Option in
m ) echo "Scenario #1: option -m- [OPTIND=${OPTIND}]";;
n | o ) echo "Scenario #2: option -$Option- [OPTIND=${OPTIND}]";;
p ) echo "Scenario #3: option -p- [OPTIND=${OPTIND}]";;
q ) echo "Scenario #4: option -q-\
with argument \"$OPTARG\" [OPTIND=${OPTIND}]";;
# Note that option 'q' must have an associated argument,
#+ otherwise it falls through to the default.
r | s ) echo "Scenario #5: option -$Option-";;
* ) echo "Unimplemented option chosen.";; # Default.
esac
done
shift $(($OPTIND - 1))
# Decrements the argument pointer so it points to next argument.
# $1 now references the first non-option item supplied on the command-line
#+ if one exists.
exit $?
# As Bill Gradwohl states,
# "The getopts mechanism allows one to specify: scriptname -mnop -mnop
#+ but there is no reliable way to differentiate what came
#+ from where by using OPTIND."
# There are, however, workarounds.
This command, when invoked from the command-line, executes a script. Within a script, a source file-name loads the file file-name. Sourcing a file (dot-command) imports code into the script, appending to the script (same effect as the #include directive in a C program). The net result is the same as if the "sourced" lines of code were physically present in the body of the script. This is useful in situations when multiple scripts use a common data file or function library.
Example 15-22. "Including" a data file
#!/bin/bash . data-file # Load a data file. # Same effect as "source data-file", but more portable. # The file "data-file" must be present in current working directory, #+ since it is referred to by its 'basename'. # Now, reference some data from that file. echo "variable1 (from data-file) = $variable1" echo "variable3 (from data-file) = $variable3" let "sum = $variable2 + $variable4" echo "Sum of variable2 + variable4 (from data-file) = $sum" echo "message1 (from data-file) is \"$message1\"" # Note: escaped quotes print_message This is the message-print function in the data-file. exit 0
File data-file for Example 15-22, above. Must be present in same directory.
# This is a data file loaded by a script.
# Files of this type may contain variables, functions, etc.
# It may be loaded with a 'source' or '.' command by a shell script.
# Let's initialize some variables.
variable1=22
variable2=474
variable3=5
variable4=97
message1="Hello, how are you?"
message2="Enough for now. Goodbye."
print_message ()
{
# Echoes any message passed to it.
if [ -z "$1" ]
then
return 1
# Error, if argument missing.
fi
echo
until [ -z "$1" ]
do
# Step through arguments passed to function.
echo -n "$1"
# Echo args one at a time, suppressing line feeds.
echo -n " "
# Insert spaces between words.
shift
# Next one.
done
echo
return 0
}
If the sourced file is itself an executable script, then it will run, then return control to the script that called it. A sourced executable script may use a return for this purpose.
Arguments may be (optionally) passed to the sourced file as positional parameters.
source $filename $arg1 arg2
It is even possible for a script to source itself, though this does not seem to have any practical applications.
Example 15-23. A (useless) script that sources itself
#!/bin/bash
# self-source.sh: a script sourcing itself "recursively."
# From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100 # Maximum number of execution passes.
echo -n "$pass_count "
# At first execution pass, this just echoes two blank spaces,
#+ since $pass_count still uninitialized.
let "pass_count += 1"
# Assumes the uninitialized variable $pass_count
#+ can be incremented the first time around.
# This works with Bash and pdksh, but
#+ it relies on non-portable (and possibly dangerous) behavior.
# Better would be to initialize $pass_count to 0 before incrementing.
while [ "$pass_count" -le $MAXPASSCNT ]
do
. $0 # Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here. Why?
done
# What occurs here is not actually recursion,
#+ since the script effectively "expands" itself, i.e.,
#+ generates a new section of code
#+ with each pass through the 'while' loop',
# with each 'source' in line 20.
#
# Of course, the script interprets each newly 'sourced' "#!" line
#+ as a comment, and not as the start of a new script.
echo
exit 0 # The net effect is counting from 1 to 100.
# Very impressive.
# Exercise:
# --------
# Write a script that uses this trick to actually do something useful.
Unconditionally terminates a script.6 The exit command may optionally take an integer argument, which is returned to the shell as the exit status of the script. It is good practice to end all but the simplest scripts with an exit 0, indicating a successful run.
If a script terminates with an
exit lacking an argument, the exit status of the script is the exit status of the last command executed in the script, not counting the exit. This is equivalent to an exit $?.
An
exit command may also be used to terminate a subshell.
This shell builtin replaces the current process with a specified command. Normally, when the shell encounters a command, it forks off a child process to actually execute the command. Using the exec builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script, therefore, it forces an exit from the script when the exec'ed command terminates.7
Example 15-24. Effects of exec
#!/bin/bash
exec echo "Exiting \"$0\"." # Exit from script here.
# ----------------------------------
# The following lines never execute.
echo "This echo will never echo."
exit 99 # This script will not exit here.
# Check exit value after script terminates
#+ with an 'echo $?'.
# It will *not* be 99.
Example 15-25. A script that exec's itself
#!/bin/bash
# self-exec.sh
# Note: Set permissions on this script to 555 or 755,
# then call it with ./self-exec.sh or sh ./self-exec.sh.
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
# Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl-C to exit ===================="
sleep 1
exec $0 # Spawns another instance of this same script
#+ that replaces the previous one.
echo "This line will never echo!" # Why not?
exit 99 # Will not exit here!
# Exit code will not be 99!
An exec also serves to reassign file descriptors. For example, exec <zzz-file replaces stdin with the file zzz-file.
The -exec option to find is not the same as the exec shell builtin.?
This command permits changing shell options on the fly (see Example 25-1 and Example 25-2). It often appears in the Bash startup files, but also has its uses in scripts. Needs version 2 or later of Bash.
shopt -s cdspell
# Allows minor misspelling of directory names with 'cd'
# Option -s sets, -u unsets.
cd /hpme # Oops! Mistyped '/home'.
pwd # /home
# The shell corrected the misspelling.
Putting a caller command inside a function echoes to stdout information about the caller of that function.
#!/bin/bash
function1 ()
{
# Inside function1 ().
caller 0 # Tell me about it.
}
function1 # Line 9 of script.
# 9 main test.sh
# ^ Line number that the function was called from.
# ^^^^ Invoked from "main" part of script.
# ^^^^^^^ Name of calling script.
caller 0 # Has no effect because it's not inside a function.
A caller command can also return caller information from a script sourced within another script. Analogous to a function, this is a "subroutine call."
You may find this command useful in debugging.
A command that returns a successful (zero) exit status, but does nothing else.
bash$ true
bash$ echo $?
0
# Endless loop while true # alias for ":" do operation-1 operation-2 ... operation-n # Need a way to break out of loop or script will hang. done
A command that returns an unsuccessful exit status, but does nothing else.
bash$ false
bash$ echo $?
1
# Testing "false" if false then echo "false evaluates \"true\"" else echo "false evaluates \"false\"" fi # false evaluates "false" # Looping while "false" (null loop) while false do # The following code will not execute. operation-1 operation-2 ... operation-n # Nothing happens! done
Similar to the which external command, type cmd identifies "cmd." Unlike which, type is a Bash builtin. The useful -a option to type identifies keywords and builtins, and also locates system commands with identical names.
bash$ type '['
[ is a shell builtin
bash$ type -a '['
[ is a shell builtin
[ is /usr/bin/[
bash$ type type
type is a shell builtin
The type command can be useful for testing whether a certain command exists.
Records the path name of specified commands -- in the shell hash table 8-- so the shell or script will not need to search the $PATH on subsequent calls to those commands. When hash is called with no arguments, it simply lists the commands that have been hashed. The -r option resets the hash table.
The bind builtin displays or modifies readline 9 key bindings.
Gets a short usage summary of a shell builtin. This is the counterpart to whatis, but for builtins. The display of help information got a much-needed update in the version 4 release of Bash.
bash$ help exit
exit: exit [n]
Exit the shell with a status of N. If N is omitted, the exit status
is that of the last command executed.
exit only terminates the process (or shell) in which it is running, not the parent process.exec is used to reassign file descriptors.
An advantage of hashing is that it is fast. A disadvantage is that collisions -- where a single key maps to more than one data item -- are possible.
For examples of hashing see Example A-20 and Example A-21.