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Bash Shell Scripting Reference Guide

She-bang Line​

  • Forces shell script to run in a specific Shell
  • Must be at beginning of first line (eg. #!/bin/bash)

Variables​

Environment​

  • System-wide or "global" variable
  • Usually appear in UPPERCASE letters
  • Can view with command: set | more
  • $ in front to expand variable to value
  • Examples: USER, PATH, HOME, SHELL

User-defined​

  • Variable created by user (command line, scripting)
  • Examples:
myVar="my value"; readonly myVar; export myVar
read -p "enter value: " myVar

Positional parameters​

  • Assign values with set command or shell script arguments
  • These variables are numbered (eg. $1, $2 ... $10}
  • Special parameters: $*, $@, $#, $$, $?

Command Substitution​

  • Useful method to expand output from a command to be used as an argument for another command.
  • Examples:
file $(ls)
set $(ls);echo $#;echo $*
echo "hostname: $(hostname)"

if / elif / else statements​

  • 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.
  • Example:
if echo $myVar | grep "match"
then
echo "Match"
fi
  • 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.
  • Example:
if [ $USER = "root" ]
then
 echo "You must be root"
 exit1
fi
  • For numberic comparison, use the test options: -gt,-ge, -lt, -le, -eq, -ne
  • Example:
if [ $grade -gt 79 ]
then
 echo "You get Good Mark"
elif [ $grade -gt 49 ]
then
 echo "You pass"
else
 echo "You fail"
fi
  • 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.
  • Examples:
if [ -d directory-pathname ]
then
echo "directory exists"
fi

if [ ! - f file-pathname ]
then
echo "File does not exist"
fi