use conditional in bash script to check string argument

Question

I am trying to write my shell script thing.sh so that upon making it an executable and running it with the single letter ``A" like so:

$ ./thing.sh A

I get the output

If argument 1 is not A, I want the output

Not A

Here is my code so far :

#!/bin/bash

if [ "$1" -eq "A"]
then echo "A"
else echo "Not A"
fi

which returns, no matter what I enter,

./thing.sh: line 3: [:missing `]'
Not A

I am trying what I hoped would check something with one or several letters and compare it against the letter A; could someone tell me what I am missing to get this to work? Thank you

Gilles Quénot · Accepted Answer · 2013-02-08 00:29:13Z

60

What about the shorter :

#!/bin/bash

[[ $1 == A ]] && echo "A" || echo "not A"

?

And a beginner version (identical logic) :

#!/bin/bash

if [[ $1 == A ]]; then
    echo "A"
else
    echo "not A"
fi

Like Scott said, you have a syntax error (missing space).

explanations

I use boolean logic here. [[ $1 == A ]] is executed, and then if its true, echo "A" is executed, and if it's false, echo "not A" is executed, See http://mywiki.wooledge.org/BashGuide/TestsAndConditionals
[[ is a bash keyword similar to (but more powerful than) the [ command. See http://mywiki.wooledge.org/BashFAQ/031 and http://mywiki.wooledge.org/BashGuide/TestsAndConditionals Unless you're writing for POSIX sh, I recommend [[.

edited Feb 8, 2013 at 0:29

answered Feb 8, 2013 at 0:17

Gilles Quénot

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3 Comments

NeilG Nov 27, 2024 at 14:40

Obviously "A" is a stand-in for something more complex and the OP puts it in quotes. Could you please clarify that lack of quotes around it in your statements? For instance the impact of spaces or at least just be explicit about the need or lack of need for quoting?

Gilles Quénot Nov 27, 2024 at 14:53

Quotes are required by POSIX. Bash's tests [[ ]] don't need it.

NeilG Nov 28, 2024 at 5:14

Presumably if the pattern that $1 was intended to match (i.e. A) included spaces then quotes would be required? For instance if A was "foo bar" then if [[ $1 == foo bar ]]; would not work? If it does work given the usual assumptions about shell scripting and coding I think a statement that it works (and hopefully why) would be great. If you could clarify this properly and also add it to the answer @Gilles_Quenot this answer would be improved.

Geoff Gustafson · Accepted Answer · 2013-02-08 00:21:16Z

6

Change the first line to:

if [ "$1" == "A" ]

The -eq operator is for integers. And as someone else mentioned, the space does matter before the ']'.

See here: http://tldp.org/LDP/abs/html/comparison-ops.html

answered Feb 8, 2013 at 0:21

Geoff Gustafson

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Comments

Scott - Слава Україні · Accepted Answer · 2013-02-08 00:17:44Z

4

Try putting a space between "A" and ].

answered Feb 8, 2013 at 0:17

Scott - Слава Україні

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Comments

user1526546 · Accepted Answer · 2013-02-08 00:18:03Z

4

You need a space after "A"

if [ "$1" -eq "A" ]

answered Feb 8, 2013 at 0:18

user1526546

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