Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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Chapter 14. Command Substitution

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.

Caution

Command substitution may result in word splitting. 
   1 COMMAND `echo a b`     # 2 args: a and b
   2 
   3 COMMAND "`echo a b`"   # 1 arg: "a b"
   4 
   5 COMMAND `echo`         # no arg
   6 
   7 COMMAND "`echo`"       # one empty arg
   8 
   9 
  10 # Thanks, S.C.

Even when there is no word splitting, command substitution can remove trailing newlines. 
   1 # cd "`pwd`"  # This should always work.
   2 # However...
   3 
   4 mkdir 'dir with trailing newline
   5 '
   6 
   7 cd 'dir with trailing newline
   8 '
   9 
  10 cd "`pwd`"  # Error message:
  11 # bash: cd: /tmp/file with trailing newline: No such file or directory
  12 
  13 cd "$PWD"   # Works fine.
  14 
  15 
  16 
  17 
  18 
  19 old_tty_setting=$(stty -g)   # Save old terminal setting.
  20 echo "Hit a key "
  21 stty -icanon -echo           # Disable "canonical" mode for terminal.
  22                              # Also, disable *local* echo.
  23 key=$(dd bs=1 count=1 2> /dev/null)   # Using 'dd' to get a keypress.
  24 stty "$old_tty_setting"      # Restore old setting. 
  25 echo "You hit ${#key} key."  # ${#variable} = number of characters in $variable
  26 #
  27 # Hit any key except RETURN, and the output is "You hit 1 key."
  28 # Hit RETURN, and it's "You hit 0 key."
  29 # The newline gets eaten in the command substitution.
  30 
  31 Thanks, S.C.

Caution

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. 
   1 dir_listing=`ls -l`
   2 echo $dir_listing     # unquoted
   3 
   4 # Expecting a nicely ordered directory listing.
   5 
   6 # However, what you get is:
   7 # total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
   8 # bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
   9 
  10 # The newlines disappeared.
  11 
  12 
  13 echo "$dir_listing"   # quoted
  14 # -rw-rw-r--    1 bozo       30 May 13 17:15 1.txt
  15 # -rw-rw-r--    1 bozo       51 May 15 20:57 t2.sh
  16 # -rwxr-xr-x    1 bozo      217 Mar  5 21:13 wi.sh

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 #  Be aware that the variables may contain embedded whitespace,
   5 #+ 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

Caution

Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke.

Example 14-1. Stupid script tricks

   1 #!/bin/bash
   2 # stupid-script-tricks.sh: Don't try this at home, folks.
   3 
   4 
   5 dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.
   6 
   7 echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
   8 # string-length of $dangerous_variable = 794151
   9 # (Does not give same count as 'wc -c /boot/vmlinuz'.)
  10 
  11 # echo "$dangerous_variable"
  12 # Don't try this! It would hang the script.
  13 
  14 
  15 #  The document author is aware of no useful applications for
  16 #+ setting a variable to the contents of a binary file.
  17 
  18 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.

   1 #include <stdio.h>
   2 
   3 /*  "Hello, world." C program  */		
   4 
   5 int main()
   6 {
   7   printf( "Hello, world." );
   8   return (0);
   9 }
 bash$ gcc -o hello hello.c
 	      

   1 #!/bin/bash
   2 # hello.sh		
   3 
   4 greeting=`./hello`
   5 echo $greeting
 bash$ sh hello.sh
 Hello, world.

Note

The $(COMMAND) form has superseded backticks for command substitution. 
   1 output=$(sed -n /"$1"/p $file)
   2 # From "grp.sh"	example.

Examples of command substitution in shell scripts:

  1. Example 10-7

  2. Example 10-24

  3. Example 9-22

  4. Example 12-2

  5. Example 12-15

  6. Example 12-12

  7. Example 12-34

  8. Example 10-12

  9. Example 10-9

  10. Example 12-24

  11. Example 16-5

  12. Example A-13

  13. Example 28-1

  14. Example 12-30

  15. Example 12-31

  16. Example 12-32

Notes

[1]

For purposes of command substitution, a command may be an external system command, an internal scripting builtin, or even a script function.

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