Command substitution reassigns the output of a command [1] or even multiple commands; it literally plugs the command output into another context. [2]
The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command-line text.
script_name=`basename $0` 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.
rm `cat filename` # "filename" contains a list of files to delete. # # S. C. points out that "arg list too long" error might result. # Better is xargs rm -- < filename # ( -- covers those cases where "filename" begins with a "-" ) textfile_listing=`ls *.txt` # Variable contains names of all *.txt files in current working directory. echo $textfile_listing textfile_listing2=$(ls *.txt) # The alternative form of command substitution. echo $textfile_listing2 # Same result. # A possible problem with putting a list of files into a single string # is that a newline may creep in. # # A safer way to assign a list of files to a parameter is with an array. # shopt -s nullglob # If no match, filename expands to nothing. # textfile_listing=( *.txt ) # # Thanks, S.C. |
Command substitution invokes a subshell. |
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.
variable1=`<file1` # Set "variable1" to contents of "file1". variable2=`cat file2` # Set "variable2" to contents of "file2". # This, however, forks a new process, #+ so the line of code executes slower than the above version. # Note that the variables may contain embedded whitespace, #+ or even (horrors), control characters. # It is not necessary to explicitly assign a variable. echo "` <$0`" # Echoes the script itself to stdout. |
# Excerpts from system file, /etc/rc.d/rc.sysinit #+ (on a Red Hat Linux installation) if [ -f /fsckoptions ]; then fsckoptions=`cat /fsckoptions` ... fi # # if [ -e "/proc/ide/${disk[$device]}/media" ] ; then hdmedia=`cat /proc/ide/${disk[$device]}/media` ... fi # # if [ ! -n "`uname -r | grep -- "-"`" ]; then ktag="`cat /proc/version`" ... fi # # if [ $usb = "1" ]; then sleep 5 mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"` kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"` ... fi |
Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop.
Example 11-2. Generating a variable from a loop
#!/bin/bash # csubloop.sh: Setting a variable to the output of a loop. variable1=`for i in 1 2 3 4 5 do echo -n "$i" # The 'echo' command is critical done` #+ to command substitution here. echo "variable1 = $variable1" # variable1 = 12345 i=0 variable2=`while [ "$i" -lt 10 ] do echo -n "$i" # Again, the necessary 'echo'. let "i += 1" # Increment. done` echo "variable2 = $variable2" # variable2 = 0123456789 # Demonstrates that it's possible to embed a loop #+ within a variable declaration. 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 $(...) form has superseded backticks for command substitution.
The $(...) form of command substitution treats a double backslash in a different way than `...`.
The $(...) form of command substitution permits nesting. [3]
Or, for something a bit more elaborate . . . Example 11-3. Finding anagrams
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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. | |
[2] | In a more technically correct sense, command substitution extracts the stdout of a command, then assigns it to a variable using the = operator. | |
[3] | In fact, nesting with backticks is also possible, but only by escaping the inner backticks, as John Default points out.
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