if

Execute the specified commands if the condition command's exit status is zero

$ if [condition_command]; then [echo "Condition is true"]; fi

$ if ! [condition_command]; then [echo "Condition is true"]; fi

$ if [condition_command]; then [echo "Condition is true"]; else [echo "Condition is false"]; fi

$ if [[ -f [path/to/file] ]]; then [echo "Condition is true"]; fi

$ if [[ -d [path/to/directory] ]]; then [echo "Condition is true"]; fi

$ if [[ -e [path/to/file_or_directory] ]]; then [echo "Condition is true"]; fi

$ if [[ -n "$[variable]" ]]; then [echo "Condition is true"]; fi

$ man test

if test-commands; then consequent-commands;
[elif more-test-commands; then more-consequents;]
[else alternate-commands;] fi

test-commands
    Commands evaluated for condition; last one's exit status (0=true) decides branch

then
    Keyword starting commands for true condition

elif
    Optional keyword for additional test-then pairs

else
    Optional keyword for commands if all prior conditions false

fi
    Required keyword ending the if block

The if statement is a core control flow construct in Unix shells like Bash and POSIX sh, enabling conditional execution of commands based on the exit status of preceding tests.

It begins with if followed by a list of test-commands separated by semicolons. The shell executes these commands, and if the final command exits with status zero (success), it runs the commands after then. Optional elif clauses provide chained conditions, else handles fallback, and fi terminates the block.

Tests typically use builtins like test (or [), [[ in Bash for string/file/arithmetic checks, or any command's exit code. Only the last test-command's status matters; short-circuiting occurs with && or ||.

Essential for scripting logic: file checks, user input validation, error handling. Supports nesting for complex flows. Readability improves with indentation, though not required. Portable across shells.

Condition relies only on final test-command's exit status, not output.
Requires proper semicolon before then/elif/else.
No implicit boolean conversion; use test or arithmetic for comparisons.
Nesting needs matching fis.

if [ -f /etc/passwd ]; then
  echo File exists;
fi

if [ "$1" = start ]; then
  service foo start;
elif [ "$1" = stop ]; then
  service foo stop;
else
  echo Usage: $0 {start|stop};
fi

Originated in Bourne shell (1977, Bell Labs). Standardized in POSIX.1-1988. Bash/Zsh extend with [[ for advanced tests.

test(1), [(1), [[(1), bash(1), sh(1)