Check out steve-parker.org/sh/tips for regular fun and tips.
Test is used by virtually every shell script written. It may not seem that way, because
test is not often called directly.
test is more frequently
[ is a symbolic link to
test, just to make shell programs more
readable. If is also normally a shell builtin (which means that the shell itself will interpret
[ as meaning test, even if your Unix environment is set up differently):
$ type [ [ is a shell builtin $ which [ /usr/bin/[ $ ls -l /usr/bin/[ lrwxrwxrwx 1 root root 4 Mar 27 2000 /usr/bin/[ -> test
This means that '[' is actually a program, just like
ls and other programs, so it must be
surrounded by spaces:
if [$foo = "bar" ]
will not work; it is interpreted as
if test$foo = "bar" ], which is a ']' without a beginning '['.
Put spaces around all your operators. I've highlighted the mandatory spaces with the word 'SPACE' - replace 'SPACE'
with an actual space; if there isn't a space there, it won't work:
if SPACE [ SPACE "$foo" SPACE = SPACE "bar" SPACE ]
Note: Some shells also accept "
==" for string comparison; this is not portable, a single "
=" should be used for strings, or "
-eq" for integers.
Test is a simple but powerful comparison utility. For full details, run
on your system, but here are some usages and typical examples.
Test is most often invoked indirectly via the
while statements. It is also the reason you will come
into difficulties if you create a program called
try to run it, as this shell builtin will be called instead of your
The syntax for
if [ ... ] then # if-code else # else-code fi
if backwards! This is used again
later with case and
Also, be aware of the syntax - the "if [ ... ]" and the "then" commands must be on different lines. Alternatively, the semicolon ";" can separate them:
if [ ... ]; then # do something fi
You can also use the
elif, like this:
if [ something ]; then echo "Something" elif [ something_else ]; then echo "Something else" else echo "None of the above" fi
This will echo "Something" if the
[ something ] test succeeds, otherwise it will test
[ something_else ], and echo "Something else" if that succeeds. If all else fails, it will echo "None of the above".
Try the following code snippet, before running it set the variable X to various values (try -1, 0, 1, hello, bye, etc). You can do this as follows (thanks to Dave for pointing out the need to export the variable, as noted in Variables - Part I.):
$ X=5 $ export X $ ./test.sh ... output of test.sh ... $ X=hello $ ./test.sh ... output of test.sh ... $ X=test.sh $ ./test.sh ... output of test.sh ...
Then try it again, with
$X as the name of an existing file, such
#!/bin/sh if [ "$X" -lt "0" ] then echo "X is less than zero" fi if [ "$X" -gt "0" ]; then echo "X is more than zero" fi [ "$X" -le "0" ] && \ echo "X is less than or equal to zero" [ "$X" -ge "0" ] && \ echo "X is more than or equal to zero" [ "$X" = "0" ] && \ echo "X is the string or number \"0\"" [ "$X" = "hello" ] && \ echo "X matches the string \"hello\"" [ "$X" != "hello" ] && \ echo "X is not the string \"hello\"" [ -n "$X" ] && \ echo "X is of nonzero length" [ -f "$X" ] && \ echo "X is the path of a real file" || \ echo "No such file: $X" [ -x "$X" ] && \ echo "X is the path of an executable file" [ "$X" -nt "/etc/passwd" ] && \ echo "X is a file which is newer than /etc/passwd"
Note that we can use the semicolon (;) to join two lines together. This
is often done to save a bit of space in simple
The backslash simply tells the shell that this is not the end of the line,
but the two (or more) lines should be treated as one. This is useful
for readability. It is customary to indent the following line.
As we see from these examples,
test can perform many tests on
numbers, strings, and filenames.
Thanks to Aaron for pointing out that
-e (both meaning "file exists"),
-S (file is a Socket),
-nt (file is newer than),
-ot (file is older than),
-ef (paths refer to the same file) and
-O (file is owned my user), are not available in the traditional Bourne shell (eg, /bin/sh on Solaris, AIX, HPUX, etc).
There is a simpler way of writing
if statements: The
|| commands give code to run if the result is true.
#!/bin/sh [ $X -ne 0 ] && echo "X isn't zero" || echo "X is zero" [ -f $X ] && echo "X is a file" || echo "X is not a file" [ -n $X ] && echo "X is of non-zero length" || \ echo "X is of zero length"
This syntax is possible because there is a file (or shell-builtin) called
[ which is linked to
test. Be careful using this
construct, though, as overuse can lead to very hard-to-read code. The
if...then...else... structure is much more readable. Use of the
[...] construct is recommended for while loops and trivial sanity
checks with which you do not want to overly distract the reader.
Note that when you set X to a non-numeric value, the first few comparisons result in the message:
test.sh: [: integer expression expected before -lt test.sh: [: integer expression expected before -gt test.sh: [: integer expression expected before -le test.sh: [: integer expression expected before -geThis is because the -lt, -gt, -le and -ge comparisons are only designed for integers, and do not work on strings. The string comparisons, such as
!=will happily treat "5" as a string, but there is no sensible way of treating "Hello" as an integer, so the integer comparisons complain.
If you want your shell script to behave more gracefully, you will have to check the contents of the variable before you test it - maybe something like this:
echo -en "Please guess the magic number: " read X echo $X | grep "[^0-9]" > /dev/null 2>&1 if [ "$?" -eq "0" ]; then # If the grep found something other than 0-9 # then it's not an integer. echo "Sorry, wanted a number" else # The grep found only 0-9, so it's an integer. # We can safely do a test on it. if [ "$X" -eq "7" ]; then echo "You entered the magic number!" fi fi
In this way you can
echo a more meaningful
message to the user, and exit gracefully. The
$? variable is explained
in Variables - Part II, and
a complicated beast, so here goes:
grep [0-9] finds lines of text which contain digits (0-9) and possibly other characters, so the caret (
grep [^0-9] finds only those lines which don't consist only of numbers. We can then take the opposite (by acting on failure, not success). Okay? The
>/dev/null 2>&1 directs any
output or errors to the special "null" device, instead of going to the user's screen.
Many thanks to Paul Schermerhorn for correcting me - this page used to claim that
grep -v [0-9] would work, but this is clearly far too simplistic.
We can use test in while loops as follows:
#!/bin/sh X=0 while [ -n "$X" ] do echo "Enter some text (RETURN to quit)" read X echo "You said: $X" done
This code will keep asking for input until you hit RETURN (X is zero length).
Thanks to Justin Heath for pointing out that the script didn't work - I'd missed out the quotes around $X in the
while [ -n "$X" ]. Without those quotes, there is nothing to test when $X is empty.
Alexander Weber has pointed out that running this script will end untidily:
$ ./test2.sh Enter some text (RETURN to quit) fred You said: fred Enter some text (RETURN to quit) wilma You said: wilma Enter some text (RETURN to quit) You said: $
This can be tidied up with another test within the loop:
#!/bin/sh X=0 while [ -n "$X" ] do echo "Enter some text (RETURN to quit)" read X if [ -n "$X" ]; then echo "You said: $X" fi done
Note also that I've used two different syntaxes for
if statements on this page. These are:
if [ "$X" -lt "0" ] then echo "X is less than zero" fi .......... and ........ if [ ! -n "$X" ]; then echo "You said: $X" fi
You must have a break between the
if statement and the
then construct. This can be a semicolon or
a newline, it doesn't matter which, but there must be one or the other between the
if and the
It would be nice to just say:
if [ ! -n "$X" ] echo "You said: $X"
fi are absolutely required.
Check out steve-parker.org/sh/tips for regular fun and tips.