Chapter 14. Command Substitution
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 $textfile_listing
# Same result.
# A possible problem with putting a list of files into a single string
# is that a newline may creep in.
#
# A safer way to assign a list of files to a parameter is with an array.
# shopt -s nullglob # If no match, filename expands to nothing.
# textfile_listing=( *.txt )
#
# Thanks, S.C. |
 | Command substitution may result in word splitting.
COMMAND `echo a b` # 2 args: a and b
COMMAND "`echo a b`" # 1 arg: "a b"
COMMAND `echo` # no arg
COMMAND "`echo`" # one empty arg
# Thanks, S.C. |
Even when there is no word splitting, command
substitution can remove trailing newlines.
# cd "`pwd`" # This should always work.
# However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$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. |
|
| 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.
dir_listing=`ls -l`
echo $dir_listing # unquoted
# Expecting a nicely ordered directory listing.
# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared.
echo "$dir_listing" # quoted
# -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 |
|
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".
# Note 1:
# Removes newlines.
#
# Note 2:
# The variables may contain embedded whitespace,
#+ or even (horrors), control characters. |
# 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 |
| 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 #!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.
dangerous_variable=`cat /boot/vmlinuz` # The compressed Linux kernel itself.
echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Does not give same count as 'wc -c /boot/vmlinuz'.)
# echo "$dangerous_variable"
# Don't try this! It would hang the script.
# The document author is aware of no useful applications for
#+ setting a variable to the contents of a binary file.
exit 0 |
Notice that a buffer overrun
does not occur. This is one instance where an interpreted
language, such as Bash, provides more protection from
programmer mistakes than a compiled language. |
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
#!/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.
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
exit 0 |
 | The $(COMMAND) form has
superseded backticks for command substitution.
output=$(sed -n /"$1"/p $file) # From "grp.sh" example.
# Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)
File_contents2=$(<$file2) # Bash permits this also. |
|
Examples of command substitution in shell scripts:
Example 10-7
Example 10-26
Example 9-26
Example 12-2
Example 12-15
Example 12-12
Example 12-39
Example 10-13
Example 10-10
Example 12-24
Example 16-7
Example A-18
Example 28-1
Example 12-32
Example 12-33
Example 12-34
