Bash-Beginners-Guide

Bash Guide for Beginners
9.7.2. Examples
A shift statement is typically used when the number of arguments to a command is not known in advance, for
instance when users can give as many arguments as they like. In such cases, the arguments are usually
processed in a while loop with a test condition of (( $# )). This condition is true as long as the number of
arguments is greater than zero. The $1 variable and the shift statement process each argument. The number of
arguments is reduced each time shift is executed and eventually becomes zero, upon which the while loop
exits.
The example below, cleanup.sh, uses shift statements to process each file in the list generated by find:
#!/bin/bash
# This script can clean up files that were last accessed over 365 days ago.
USAGE="Usage: $0 dir1 dir2 dir3 ... dirN"
if [ "$#" == "0" ]; then
echo "$USAGE"
exit 1
fi
while (( "$#" )); do
if [[ $(ls "$1") == "" ]]; then
echo "Empty directory, nothing to be done."
else
find "$1" -type f -a -atime +365 -exec rm -i {} \;
fi
shift
done
-exec vs. xargs
The above find command can be replaced with the following:
find options | xargs [commands_to_execute_on_found_files]
The xargs command builds and executes command lines from standard input. This has the advantage
that the command line is filled until the system limit is reached. Only then will the command to execute
be called, in the above example this would be rm. If there are more arguments, a new command line will
be used, until that one is full or until there are no more arguments. The same thing using find -exec
calls on the command to execute on the found files every time a file is found. Thus, using xargs greatly
speeds up your scripts and the performance of your machine.
In the next example, we modified the script from Section 8.2.4.4 so that it accepts multiple packages to install
at once:
#!/bin/bash
if [ $# -lt 1 ]; then
echo "Usage: $0 package(s)"
exit 1
fi
while (($#)); do
yum install "$1"