Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
let "a −= 5" # Equivalent to let "a = a − 5"
echo "32 − 5 = $a"
let "a = a * 10" # Equivalent to let "a = a * 10"
echo "27 * 10 = $a"
let "a %= 8" # Equivalent to let "a = a % 8"
echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"
echo
eval
exit 0
eval arg1 [arg2] ... [argN]
Translates into commands the arguments in a list (useful for code generation within a script).
Example 11−9. Showing the effect of eval
#!/bin/bash
y=`eval ls −l` # Similar to y=`ls −l`
echo $y
# but linefeeds removed because "echoed" variable is unquoted.
echo
echo "$y" # Linefeeds preserved when variable is quoted.
echo; echo
y=`eval df`
echo $y
# Similar to y=`df`
# but linefeeds removed.
# When LF's not preserved, it may make it easier to parse output,
#+ using utilities such as "awk".
exit 0
Example 11−10. Forcing a log−off
#!/bin/bash
y=`eval ps ax | sed −n '/ppp/p' | awk '{ print $1 }'`
# Finding the process number of 'ppp'.
kill −9 $y
# Killing it
# Above lines may be replaced by
# kill −9 `ps ax | awk '/ppp/ { print $1 }'
chmod 666 /dev/ttyS3
# Doing a SIGKILL on ppp changes the permissions
# on the serial port. Restore them to previous state.
rm /var/lock/LCK..ttyS3
# Remove the serial port lock file.
exit 0
Chapter 11. Internal Commands and Builtins 133