Advanced Bash−Scripting Guide

Chapter 14. Command Substitution
Command substitution reassigns the output of a command [44] 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."
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)
echo $textfile_listing
# Same result.
# The alternative form of command substitution.
# 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`
COMMAND "`echo`"
# no arg
# 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
Chapter 14. Command Substitution 236