checking for string in variable

Question

In bash I am often checking the value of a variable, is it OK that it do it this way ? Is there a better way?

Example:

    if [ "`echo ${file} | grep -vE '(Trash|home)'`" ] ;then

Which is checking that the variable file does NOT contain the words Trash or home, if it does not then do something. The point is that I'm using echo and grep, probably wasteful I assume.

Another example:

    if [ "`less ${TEMPQUERY} | grep 'http'`" ] ;then

Which is checking a file for the string http, if it is in the file, do something.

I guess I'm just wondering what other people do and if there is some strong reason why I should not be coding things in such a way. Anyway, Thank you in advance for you time.

you may want to check -q option of grep also check this question for string contains check: stackoverflow.com/questions/229551/string-contains-in-bash for file check, you don't need less, just grep -q 'http' file and read the return value. — Kent
– Kent, Commented Jan 11, 2013 at 19:41

Ignacio Vazquez-Abrams · Accepted Answer · 2013-01-11 19:51:25Z

5

$ file="My home"
$ [[ $file =~ Trash|home ]] ; echo $?
0
$ file="root"
$ [[ $file =~ Trash|home ]] ; echo $?
1

$ cat input.txt
http://...
$ grep -v -q http input.txt ; echo $?
1

answered Jan 11, 2013 at 19:51

Ignacio Vazquez-Abrams

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3 Comments

chepner Over a year ago

+1 The main point is, don't use grep to do string operations that bash can do natively.

glenn jackman Over a year ago

I would think that don't use regular expressions when glob patterns will do: if [[ $file != *Trash* && $file != *home* ]]; then ...

Gordon Davisson Over a year ago

Note that [[ ... ]] is a bashism; be sure to start your script with #!/bin/bash (or #!/usr/bin/env bash), not #!/bin/sh

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