Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
#!/bin/bash
echo $_
du >/dev/null
echo $_
ls −al >/dev/null
echo $_
# /bin/bash
# Just called /bin/bash to run the script.
# So no output from command.
# du
# So no output from command.
# −al (last argument)
$?
$$
:
echo $_ # :
Exit status of a command, function, or the script itself (see Example 23−3)
Process id of the script itself. The $$ variable often finds use in scripts to construct "unique" temp file
names (see Example A−14, Example 30−6, Example 12−23, and Example 11−23). This is usually
simpler than invoking mktemp.
9.2. Manipulating Strings
Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified
focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr
command. This results in inconsistent command syntax and overlap of functionality, not to mention
confusion.
String Length
${#string}
expr length $string
expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ} # 15
echo `expr length $stringZ` # 15
echo `expr "$stringZ" : '.*'` # 15
Example 9−10. Inserting a blank line between paragraphs in a text file
#!/bin/bash
# paragraph−space.sh
# Inserts a blank line between paragraphs of a single−spaced text file.
# Usage: $0