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{{Admon/note|Bash Shell Scripting Reference Guide:|<br>'''<u>She-bang Line</u>'''<ul><li>Forces shell script to run in a specific Shell</li><li>Must be at beginning of first line (eg. '''#!/bin/bash''')<br><br></li></ul>'''<u>Variables</u>'''<blockquote>'''Environment'''<ul><li>System-wide or "global" variable</li><li>Usually appear in UPPERCASE letters</li><li>Can view with command: '''set &#124; more'''</li><li>'''$''' in front to expand variable to value<li>Examples: '''USER''', '''PATH''', '''HOME''', '''SHELL'''</li></ul></blockquote><blockquote>'''User-defined''' <ul><li>Variable created by user (command line, scripting)</li><li>Examples:<br>''myVar&#61;"my value"; readonly myVar; export myVar''<br>''read -p "enter value: " myVar''</li></ul></blockquote><blockquote>'''Positional parameters'''<ul><li>Assign values with set command or shell script arguments</li><li>These variables are numbered (eg. $1, $2 ... $10}</li><li>Special parameters: $*, $@, $#, $$, $?<br></li></ul></blockquote>'''<u>Command Substitution</u>'''<ul><li>Useful method to expand output from a command to be used as an argument for another command.</li><li>Examples:<br>''file $(ls)''<br>''set $(ls);echo $#;echo $*''<br>''echo "hostname: $(hostname)"''<br><br></li></ul>'''<u>if / elif / else statements</u>'''<ul><li>If a command runs (even pipeline command like to grep to match) will be true (0); otherwise, false (non-zero), thus can use with logic statements.</li>Example:<br>''if echo $myVar &#124; grep "match"''<br>''then''<br>''echo "Match"''<br>''fi''<br></li><li>The '''test''' command is used to test conditions. Square brackets '''[ ]''' is short-cut for test command (args contained inside with spaces). The '''exit''' command can be used to terminate the shell script with a false value.<br>Example:<br>''if [ $USER &#61; "root" ]''<br>''then''<br>&nbsp;''echo "You must be root" ''<br>&nbsp;''exit1''<br>''fi''<br></li><li>For numberic comparison, use the '''test options''': '''-gt''','''-ge''', '''-lt''', '''-le''', '''-eq''', '''-ne'''<br>Example:<br>''if [ $grade -gt 79 ]''<br>''then''<br>&nbsp;''echo "You get Good Mark"''<br>''elif [ $grade -gt 49 ]''<br>''then''<br>&nbsp;''echo "You pass"''<br>''else''<br>&nbsp;''echo "You fail"''<br>''fi''<br></li><li>For testing for file information, you can use '''-d''' to test if directory pathname exists, and '''-f''' if the file pathname exists. You can use '''!''' for negation.<br>Examples:<br>''if [ -d directory-pathname ]''<br>''then''<br> ''echo "directory exists"''<br>''fi''<br><br>''if [ ! - f file-pathname ]''<br>''then''<br> ''echo "File does not exist"''<br>''fi''</li></ul>}}