An “and” operator for an “if” statement in Bash

221
votes

I'm trying to create a simple Bash script to check if the website is down and for some reason the "and" operator doesn't work:

#!/usr/bin/env bash

WEBSITE=domain.com
SUBJECT="$WEBSITE DOWN!"
EMAILID="[email protected]"
STATUS=$(curl -sI $WEBSITE | awk '/HTTP\/1.1/ { print $2 }')
STRING=$(curl -s $WEBSITE | grep -o "string_to_search")
VALUE="string_to_search"

if [ $STATUS -ne 200 ] && [[ "$STRING" != "$VALUE" ]]; then
    echo "Website: $WEBSITE is down, status code: '$STATUS' - $(date)" | mail -s "$SUBJECT" $EMAILID
fi

The "-a" operator also doesn't work:

if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]

Could you also please advise when to use:

  • single and double square brackets
  • parenthesis

?

4
bashif-statement
Could you please be more precise as to what "doesn't work" ? Do you have a specific error message, or does is simply not provide the expected output ? – Julien Vivenot
I was actually receiving "unary operator expected" so it looks like quoting helps – HTF
-a has duplicity. When used with the Bourne shell style test command, a.k.a. [, the it means and. When used as a conditional expression then it is testing to see if a file exists. Yes it is confusing, best avoided. – cdarke
Check this out:theunixshell.blogspot.com/2013/05/… – Vijay

4 Answers

327
votes

What you have should work, unless ${STATUS} is empty. It would probably be better to do:

if ! [ "${STATUS}" -eq 200 ] 2> /dev/null && [ "${STRING}" != "${VALUE}" ]; then

or

if [ "${STATUS}" != 200 ] && [ "${STRING}" != "${VALUE}" ]; then

It's hard to say, since you haven't shown us exactly what is going wrong with your script.

Personal opinion: never use [[. It suppresses important error messages and is not portable to different shells.

52
votes

Try this:

if [ ${STATUS} -ne 100 -a "${STRING}" = "${VALUE}" ]

or

if [ ${STATUS} -ne 100 ] && [ "${STRING}" = "${VALUE}" ]
29
votes

Try this:

if [ $STATUS -ne 200 -a "$STRING" != "$VALUE" ]; then
14
votes

Quote:

The "-a" operator also doesn't work:

if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]

For a more elaborate explanation: [ and ] are not Bash reserved words. The if keyword introduces a conditional to be evaluated by a job (the conditional is true if the job's return value is 0 or false otherwise).

For trivial tests, there is the test program (man test).

As some find lines like if test -f filename; then foo bar; fi, etc. annoying, on most systems you find a program called [ which is in fact only a symlink to the test program. When test is called as [, you have to add ] as the last positional argument.

So if test -f filename is basically the same (in terms of processes spawned) as if [ -f filename ]. In both cases the test program will be started, and both processes should behave identically.

Here's your mistake: if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]] will parse to if + some job, the job being everything except the if itself. The job is only a simple command (Bash speak for something which results in a single process), which means the first word ([) is the command and the rest its positional arguments. There are remaining arguments after the first ].

Also not, [[ is indeed a Bash keyword, but in this case it's only parsed as a normal command argument, because it's not at the front of the command.