Linux Know-How: Introduction to if

At times you need to specify different courses of action to be taken in a shell script, depending on the success or failure of a command. The if construction allows you to specify such conditions.

The TEST-COMMAND list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

The TEST-COMMAND often involves numerical or string comparison tests, but it can also be any command that returns a status of zero when it succeeds and some other status when it fails. Unary expressions are often used to examine the status of a file. If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor "N" is checked. stdin, stdout and stderr and their respective file descriptors may also be used for tests.

Expressions used with if

The table below contains an overview of the so-called "primaries" that make up the TEST-COMMAND command or list of commands. These primaries are put between square brackets to indicate the test of a conditional expression.

Table 7-1. Primary expressions

Primary	Meaning
[ `-a` `FILE` ]	True if `FILE` exists.
[ `-b` `FILE` ]	True if `FILE` exists and is a block-special file.
[ `-c` `FILE` ]	True if `FILE` exists and is a character-special file.
[ `-d` `FILE` ]	True if `FILE` exists and is a directory.
[ `-e` `FILE` ]	True if `FILE` exists.
[ `-f` `FILE` ]	True if `FILE` exists and is a regular file.
[ `-g` `FILE` ]	True if `FILE` exists and its SGID bit is set.
[ `-h` `FILE` ]	True if `FILE` exists and is a symbolic link.
[ `-k` `FILE` ]	True if `FILE` exists and its sticky bit is set.
[ `-p` `FILE` ]	True if `FILE` exists and is a named pipe (FIFO).
[ `-r` `FILE` ]	True if `FILE` exists and is readable.
[ `-s` `FILE` ]	True if `FILE` exists and has a size greater than zero.
[ `-t` `FD` ]	True if file descriptor `FD` is open and refers to a terminal.
[ `-u` `FILE` ]	True if `FILE` exists and its SUID (set user ID) bit is set.
[ `-w` `FILE` ]	True if `FILE` True if FILE exists and is writable.
[ `-x` `FILE` ]	True if `FILE` exists and is executable.
[ `-O` `FILE` ]	True if `FILE` exists and is owned by the effective user ID.
[ `-G` `FILE` ]	True if `FILE` exists and is owned by the effective group ID.
[ `-L` `FILE` ]	True if `FILE` exists and is a symbolic link.
[ `-N` `FILE` ]	True if `FILE` exists and has been modified since it was last read.
[ `-S` `FILE` ]	True if `FILE` exists and is a socket.
[ `FILE1` `-nt` `FILE2` ]	True if `FILE1` has been changed more recently than `FILE2`, or if `FILE1` exists and `FILE2` does not.
[ `FILE1` `-ot` `FILE2` ]	True if `FILE1` is older than `FILE2`, or is `FILE2` exists and `FILE1` does not.
[ `FILE1` `-ef` `FILE2` ]	True if `FILE1` and `FILE2` refer to the same device and inode numbers.
[ `-o` OPTIONNAME ]	True if shell option "OPTIONNAME" is enabled.
`[ -z` STRING ]	True of the length of "STRING" is zero.
`[ -n` STRING ] or [ STRING ]	True of the length of "STRING" is non-zero.
[ STRING1 == STRING2 ]	True if the strings are equal. "=" may be used instead of "==" for strict POSIX compliance.
[ STRING1 != STRING2 ]	True if the strings are not equal.
[ STRING1 < STRING2 ]	True if "STRING1" sorts before "STRING2" lexicographically in the current locale.
[ STRING1 > STRING2 ]	True if "STRING1" sorts after "STRING2" lexicographically in the current locale.
[ ARG1 OP ARG2 ]	"OP" is one of `-eq`, `-ne`, `-lt`, `-le`, `-gt` or `-ge`. These arithmetic binary operators return true if "ARG1" is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to "ARG2", respectively. "ARG1" and "ARG2" are integers.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

Table 7-2. Combining expressions

Operation	Effect
[ ! EXPR ]	True if EXPR is false.
[ ( EXPR ) ]	Returns the value of EXPR. This may be used to override the normal precedence of operators.
[ EXPR1 -a EXPR2 ]	True if both EXPR1 and EXPR2 are true.
[ EXPR1 -o EXPR2 ]	True if either EXPR1 or EXPR2 is true.

The [ (or test) built-in evaluates conditional expressions using a set of rules based on the number of arguments. More information about this subject can be found in the Bash documentation. Just like the if is closed with fi, the opening angular bracket should be closed after the conditions have been listed.

Commands following the then statement

The CONSEQUENT-COMMANDS list that follows the then statement can be any valid UNIX command, any executable program, any executable shell script or any shell statement, with the exception of the closing fi. It is important to remember that the then and fi are considered to be separated statements in the shell. Therefore, when issued on the command line, they are separated by a semi-colon.

In a script, the different parts of the if statement are usually well-separated. Below, a couple of simple examples.

Checking files

The first example checks for the existence of a file:

anny ~> cat msgcheck.sh
#!/bin/bash

echo "This scripts checks the existence of the messages file."
echo "Checking..."
if [ -f /var/log/messages ]
  then
    echo "/var/log/messages exists."
fi
echo
echo "...done."

anny ~> ./msgcheck.sh
This scripts checks the existence of the messages file.
Checking...
/var/log/messages exists.

...done.

Checking shell options

To add in your Bash configuration files:

# These lines will print a message if the noclobber option is set:

if [ -o noclobber ]
  then
                echo "Your files are protected against accidental overwriting using redirection."
fi

The environment

The above example will work when entered on the command line:

anny ~> if [ -o noclobber ] ; then echo ; echo "your files are protected
against overwriting." ; echo ; fi

your files are protected against overwriting.

anny ~>

However, if you use testing of conditions that depend on the environment, you might get different results when you enter the same command in a script, because the script will open a new shell, in which expected variables and options might not be set automatically.