XXX Chapter 12. Substitució de comanda

Command substitution reassigns the output of a command ` [1] <commandsub.html#FTN.AEN7205>`__ or even multiple commands; it
literally plugs the command output into another context. ` [2]

<commandsub.html#FTN.AEN7211>`__

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 <loops1.html#FORLOOPREF1>`__ 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 <quotingvar.html#WSPLITREF>`__ .

 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 trai
ling newline

‘

cd ‘dir with trailin

g newline

‘

cd “pwd” # Error

message:
# bash: cd: /tmp/fil
e with trailing newline:
No such file or directo

ry

cd “$PWD” # Works

fine.

old_tty_setting=$(st

ty -g) # Save old term inal setting.

echo “Hit a key ” stty -icanon -echo

# Disable “cano

nical” mode for terminal .

# Also, disable
local echo.
key=$(dd bs=1 count=

1 2> /dev/null) # Usin g ‘dd’ to get a keypress .

stty “$old_tty_setti

ng” # Restore old s etting.

echo “You hit ${#key

} key.” # ${#variable} = number of characters i n $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 e

aten in the command subs titution.

#Code snippet by StÃ

©phane Chazelas.

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.

# 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.

#Code snippet by Stéphane Chazelas.

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.

# 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.

#Code snippet by Stéphane Chazelas.

|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.

   dir_listing=`ls -l`
   echo $dir_listing
 # unquoted

   # Expecting a nicely
ordered directory listi

ng.

# However, what you
get is:
# total 3 -rw-rw-r–

1 bozo bozo 30 May 13 1

7:15 1.txt -rw-rw-r– 1 bozo

# bozo 51 May 15 20:

57 t2.sh -rwxr-xr-x 1 bo zo bozo 217 Mar 5 21:13 wi.sh

# The newlines disap

peared.

echo “$dir_listing”
# quoted
# -rw-rw-r– 1 bo
zo 30 May 13 17:15
1.txt
# -rw-rw-r– 1 bo
zo 51 May 15 20:57
t2.sh
# -rwxr-xr-x 1 bo
zo 217 Mar 5 21:13
wi.sh
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

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 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 -rgrep -- "-"`" ]; 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

|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.

Exemple 1. Stupid script tricks

#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.

exit 99  ### Comment out this line if you dare.

dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Newer kernels are bigger.)
# 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.

#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.

exit 99  ### Comment out this line if you dare.

dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Newer kernels are bigger.)
# 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

#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.

exit 99  ### Comment out this line if you dare.

dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Newer kernels are bigger.)
# 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

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.

Exemple 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
#+ inside 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.\n" );
  return (0);
}

bash$ gcc -o hello hello.c

#!/bin/bash
# hello.sh

greeting=`./hello`
echo $greeting

bash$ sh hello.sh
Hello, world.

#include <stdio.h>

/*  "Hello, world." C program  */

int main()
{
  printf( "Hello, world.\n" );
  return (0);
}

bash$ gcc -o hello hello.c

#!/bin/bash
# hello.sh

greeting=`./hello`
echo $greeting

bash$ sh hello.sh
Hello, world.

#include <stdio.h>

/*  "Hello, world." C program  */

int main()
{
  printf( "Hello, world.\n" );
  return (0);
}

bash$ gcc -o hello hello.c

#!/bin/bash
# hello.sh

greeting=`./hello`
echo $greeting

bash$ sh hello.sh
Hello, world.

|Note

The $(...) form has superseded backticks for command substitution.
output=$(sed -n /"$1

“/p $file) # From “grp .sh” example.

# Setting a variable

to the contents of a te

xt file.
File_contents1=$(cat
$file1)
File_contents2=$(<$f

ile2) # Bash perm its this also.

The $(...) form of command substitution treats a double backslash in a different way than `...` .

bash$ echo `echo \\`


bash$ echo $(echo \\
)
The $(...) form of command substitution permits nesting. ` [3]
<commandsub.html#FTN.AEN7308>`__
word_count=$( wc -w

$(echo * | awk ‘{print $ 8}’) )

Or, for something a bit more elaborate . . .

Exemple 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.
#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
#  http://bash.deta.in/yawl-0.3.2.tar.gz

E_NOARGS=86
E_BADARG=87
MINLEN=7

if [ -z "$1" ]
then
  echo "Usage $0 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 $1grep $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 exercise in anagram finding.

exit $?

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.

bash$ echo `echo \\`


bash$ echo $(echo \\)
\

word_count=$( wc -w $(echo *awk '{print $8}') )

#!/bin/bash
# agram2.sh
# Example of nested command substitution.

#  Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
#  http://bash.deta.in/yawl-0.3.2.tar.gz

E_NOARGS=86
E_BADARG=87
MINLEN=7

if [ -z "$1" ]
then
  echo "Usage $0 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 $1grep $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 exercise in anagram finding.

exit $?

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.

bash$ echo `echo \\`


bash$ echo $(echo \\)
\

word_count=$( wc -w $(echo *awk '{print $8}') )

#!/bin/bash
# agram2.sh
# Example of nested command substitution.

#  Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
#  http://bash.deta.in/yawl-0.3.2.tar.gz

E_NOARGS=86
E_BADARG=87
MINLEN=7

if [ -z "$1" ]
then
  echo "Usage $0 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 $1grep $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 exercise in anagram finding.

exit $?

Examples of command substitution in shell scripts:

  1. Example 11-8
  2. Example 11-27
  3. Example 9-16
  4. Example 16-3
  5. Example 16-22
  6. Example 16-17
  7. Example 16-54
  8. Example 11-14
  9. Example 11-11
  10. Example 16-32
  11. Example 20-8
  12. Example A-16
  13. Example 29-3
  14. Example 16-47
  15. Example 16-48
  16. Example 16-49

Notes

` [1] <commandsub.html#AEN7205>`__

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

` [2] <commandsub.html#AEN7211>`__

In a more technically correct sense, command substitution extracts the `` stdout `` of a command, then assigns it to a variable using the = operator.

` [3] <commandsub.html#AEN7308>`__

In fact, nesting with backticks is also possible, but only by escaping the inner backticks, as John Default points out.

word_count=` wc -w \

`echo * | awk ‘{print $8 }’` `

word_count=` wc -w \`echo *awk '{print $8}'\` `

word_count=` wc -w \`echo *awk '{print $8}'\` `