Linux Pocket Guide

test and “[”

The test command (built into the shell) will evaluate simple Boolean expressions involving numbers and strings, setting its exit status to 0 (true) or 1 (false):

$ test 10 -lt 5       Is 10 less than 5?
$ echo $?
1                     No, it isn't
$ test -n "hello"     Does the string "hello" have nonzero length?
$ echo $?
0                     Yes, it does

A list of common test arguments are found in Table 1-12, for checking properties of integers, strings, and files.

test has an unusual alias, “[” (left square bracket), as a shorthand for use with conditionals and loops. If you use this shorthand, you must supply a final argument of “]” (right square bracket) to signify the end of the test. The following tests are identical to those before:

$ [ 10 -lt 5 ]
$ echo $?
1
$ [ -n "hello" ]
$ echo $?
0

Remember that “[” is a command like any other, so it is followed by individual arguments separated by whitespace. So if you mistakenly forget some whitespace:

$ [ 5 -lt 4]          No space between 4 and ]
bash: [: missing ']'

then test thinks the final argument is the string “4]” and complains that the final bracket is missing.

Table 1-12. Some common arguments for the test command

File tests
-d name	File name is a directory
-f name	File name is a regular file
-L name	File name is a symbolic link
-r name	File name exists and is readable
-w name	File name exists and is writable
-x name	File name exists and is executable
-s name	File name exists and its size is nonzero
f1 -nt f2	File f1 is newer than file f2
f1 -ot f2	File f1 is older than file f2
String tests ...