Shell Script Conditionals

1. If else

  • if : Executes commands if the exit code of a command is 0
  • if-else : Provides an alternative if condition fails:
  • if-elif-else : Tests multiple conditions

2. test / [ ] operators

These are same : 

if test –f /tmp/foo; then
if [ -f /tmp/foo ]; then

File and directory checks: 

[ -f /tmp/foo ]    # file exists
[ ! -f /tmp/foo ]  # file not exists
[ -d /tmp ]        # directory exists
[ -x `which ls` ]  # executable exists
Numeric and string comparisons:

[ 5 -eq 5 ]       # numeric equality
[ !5 -eq 5 ]      # numeric not equality
[ 5 -ne 5 ]       # numeric inequality

[ "a" = "a" ]     # string equality
[ "a" != "a" ]    # string different

[ 1 -ne 2 -a 3 -eq 3 ] # AND :  both must be the same
[ 1 -eq 1 -o 2 -eq 3 ] # OR :  either can be the same

3. case statement

  • Pattern matching alternative to multiple elifs:

  • | → OR multiple options

  • * → default (else)
  • ;; → end of each block

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