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Linux Pocket Guide
book

Linux Pocket Guide

by Daniel J. Barrett
February 2004
Beginner content levelBeginner
200 pages
5h 40m
English
O'Reilly Media, Inc.
Content preview from 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 ...

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Publisher Resources

ISBN: 9780596806347Errata Page