Command
substitution reassigns the output of a command
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.
script_name=`basename $0`
echo "The name of this script is $script_name." |
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_listing
# 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 may result in word splitting.
COMMAND `echo a b` # 2 args: a and b
COMMAND "`echo a b`" # 1 arg: "a b"
COMMAND `echo` # no arg
COMMAND "`echo`" # one empty arg
# Thanks, S.C. |
|
| Word splitting resulting from command
substitution may remove trailing newlines characters
from the output of the reassigned command(s). This can
cause unpleasant surprises.
dir_listing=`ls -l`
echo $dirlisting
# Expecting a nicely ordered directory listing, such as:
# -rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt
# -rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh
# -rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh
# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared. |
Even when there is no word splitting, command
substitution can remove trailing newlines.
# cd "`pwd`" # This should always work.
# However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD" # Works fine.
old_tty_setting=$(stty -g) # Save old terminal setting.
echo "Hit a key "
stty -icanon -echo # Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress.
stty "$old_tty_setting" # Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.
Thanks, S.C. |
|
 | 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".
# Be aware that the variables may contain embedded whitespace,
#+ or even (horrors), control characters. |
|
 | The $(COMMAND) form has
superseded backticks for command substitution.
output=$(sed -n /"$1"/p $file)
# From "grp.sh" example. |
|
Examples of command substitution in shell scripts:
Example 10-7
Example 10-24
Example 9-20
Example 12-2
Example 12-14
Example 12-11
Example 12-29
Example 10-12
Example 10-9
Example 12-23
Example 16-5
Example A-12
Example 28-1
Example 12-27
Example 12-28