Bash Shell Reference Guide

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

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