Command substitution reassigns the output of a command [1] or even multiple commands; it literally plugs the command output into another context.
The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command line text.
1 script_name=`basename $0` 2 echo "The name of this script is $script_name." |
The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for loop.
1 rm `cat filename` # "filename" contains a list of files to delete. 2 # 3 # S. C. points out that "arg list too long" error might result. 4 # Better is xargs rm -- < filename 5 # ( -- covers those cases where "filename" begins with a "-" ) 6 7 textfile_listing=`ls *.txt` 8 # Variable contains names of all *.txt files in current working directory. 9 echo $textfile_listing 10 11 textfile_listing2=$(ls *.txt) # The alternative form of command substitution. 12 echo $textfile_listing 13 # Same result. 14 15 # A possible problem with putting a list of files into a single string 16 # is that a newline may creep in. 17 # 18 # A safer way to assign a list of files to a parameter is with an array. 19 # shopt -s nullglob # If no match, filename expands to nothing. 20 # textfile_listing=( *.txt ) 21 # 22 # Thanks, S.C. |
Command substitution may result in word splitting.
Even when there is no word splitting, command substitution can remove trailing newlines.
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Using echo to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises.
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Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command.
1 variable1=`<file1` # Set "variable1" to contents of "file1". 2 variable2=`cat file2` # Set "variable2" to contents of "file2". 3 4 # Note 1: 5 # Removes newlines. 6 # 7 # Note 2: 8 # The variables may contain embedded whitespace, 9 #+ or even (horrors), control characters. |
1 # Excerpts from system file, /etc/rc.d/rc.sysinit 2 #+ (on a Red Hat Linux installation) 3 4 5 if [ -f /fsckoptions ]; then 6 fsckoptions=`cat /fsckoptions` 7 ... 8 fi 9 # 10 # 11 if [ -e "/proc/ide/${disk[$device]}/media" ] ; then 12 hdmedia=`cat /proc/ide/${disk[$device]}/media` 13 ... 14 fi 15 # 16 # 17 if [ ! -n "`uname -r | grep -- "-"`" ]; then 18 ktag="`cat /proc/version`" 19 ... 20 fi 21 # 22 # 23 if [ $usb = "1" ]; then 24 sleep 5 25 mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"` 26 kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"` 27 ... 28 fi |
Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop.
Example 14-2. Generating a variable from a loop
1 #!/bin/bash 2 # csubloop.sh: Setting a variable to the output of a loop. 3 4 variable1=`for i in 1 2 3 4 5 5 do 6 echo -n "$i" # The 'echo' command is critical 7 done` #+ to command substitution. 8 9 echo "variable1 = $variable1" # variable1 = 12345 10 11 12 i=0 13 variable2=`while [ "$i" -lt 10 ] 14 do 15 echo -n "$i" # Again, the necessary 'echo'. 16 let "i += 1" # Increment. 17 done` 18 19 echo "variable2 = $variable2" # variable2 = 0123456789 20 21 exit 0 |
Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of writing a program or script that outputs to stdout (like a well-behaved UNIX tool should) and assigning that output to a variable.
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The $(COMMAND) form has superseded backticks for command substitution.
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Examples of command substitution in shell scripts:
[1] | For purposes of command substitution, a command may be an external system command, an internal scripting builtin, or even a script function. |