|
|
||
Chapter 14. Command SubstitutionCommand 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ドル` 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 $textfile_listing2 # Same result. # A possible problem with putting a list of files into a single string # is that a newline may creep in. # # A safer way to assign a list of files to a parameter is with an array. # shopt -s nullglob # If no match, filename expands to nothing. # textfile_listing=( *.txt ) # # Thanks, S.C. Note Command substitution invokes a subshell. Caution Command substitution may result in word splitting. COMMAND `echo a b` # 2 args: a and b COMMAND "`echo a b`" # 1 arg: "a b" COMMAND `echo` # no arg COMMAND "`echo`" # one empty arg # Thanks, S.C. Even when there is no word splitting, command substitution can remove trailing newlines. # cd "`pwd`" # This should always work.
# However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$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". # This, however, forks a new process, #+ so the line of code executes slower than the above version. # Note: # 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"`
...
fiDo 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 0Notice 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 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 = $variable2" # variable2 = 0123456789 # Demonstrates that it's possible to embed a loop #+ within a variable declaration. exit 0
Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of writing a program or script that outputs to stdout (like a well-behaved UNIX tool should) and assigning that output to a variable.
#include <stdio.h>
/* "Hello, world." C program */
int main()
{
printf( "Hello, world." );
return (0);
}bash$ gcc -o hello hello.c
#!/bin/bash # hello.sh greeting=`./hello` echo $greeting
bash$ sh hello.sh Hello, world.
The $(COMMAND) form has superseded backticks for command substitution.
output=$(sed -n /"1ドル"/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.
The $(...) form of command substitution treats a double backslash in a different way than `...`.
bash$ echo `echo \\` bash$ echo $(echo \\) \
The $(...) form of command substitution permits nesting. [3]
word_count=$( wc -w $(ls -l | awk '{print 9ドル}') )Or, for something a bit more elaborate . . .
Example 14-3. Finding anagrams
#!/bin/bash
# agram2.sh
# Example of nested command substitution.
# Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
# https://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
# https://personal.riverusers.com/~thegrendel/yawl-0.3.2.tar.gz
E_NOARGS=66
E_BADARG=67
MINLEN=7
if [ -z "1ドル" ]
then
echo "Usage 0ドル LETTERSET"
exit $E_NOARGS # Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
echo "Argument must have at least $MINLEN letters."
exit $E_BADARG
fi
FILTER='.......' # Must have at least 7 letters.
# 1234567
Anagrams=( $(echo $(anagram 1ドル | grep $FILTER) ) )
# | | nested command sub. | |
# ( array assignment )
echo
echo "${#Anagrams[*]} 7+ letter anagrams found"
echo
echo ${Anagrams[0]} # First anagram.
echo ${Anagrams[1]} # Second anagram.
# Etc.
# echo "${Anagrams[*]}" # To list all the anagrams in a single line . . .
# Look ahead to the "Arrays" chapter for enlightenment on
#+ what's going on here.
# See also the agram.sh script for an example of anagram finding.
exit $?Examples of command substitution in shell scripts:
For purposes of command substitution, a command may be an external system command, an internal scripting builtin, or even a script function.
In a more technically correct sense, command substitution extracts the stdout of a command, then assigns it to a variable using the = operator.
In fact, nesting with backticks is also possible, but only by escaping the inner backticks, as John Default points out.
word_count=` wc -w \`ls -l | awk '{print 9ドル}'\` `