Error when checking for substring in Bash

Question

I'm quite new to shell scripting and have encountered an issue when trying to check for substrings within a string. I want to build code that checks if you are running a 64bit-based system. This is indicated by the output of the uname -m && cat /etc/*release command by the x86_64 in the first line.

Here's my code:

INFO=$(uname -m && cat /etc/*release)
if [ "$INFO" == *"x86_64"* ]
then
    echo "You are running a 64bit-based system!"
else
    echo "Your system architecture is wrong!"
    exit
fi

Although I run a 64-bit based system and the x86_64 shows up in the output of my command, the if statement still returns false, so I get the output Your system architecture is wrong!. It should be the opposite. Can someone help me out by identifying what I did wrong? I also accept general suggestions for improving my approach, but in the first place, I'd like to know where the bug is.

Many thanks for your help!

Note that Arch Linux's `/etc/arch_release` doesn't contain the string `x86` or `x86_64`. (It has some lines like `PRETTY_NAME="Arch Linux"` and `BUILD_ID=rolling`). Perhaps `file /bin/ls` would be more portable as a way to check the user-space bitness. (As opposed to kernel bitness with `uname -a`, which might be a 64-bit kernel running 32-bit user-space.) — Peter Cordes, Jun 17 '20 at 15:02
Are you running this script with `/bin/sh` or with `/bin/bash` or something else? — glenn jackman, Jun 17 '20 at 15:25

score 4 · Accepted Answer · answered Jun 17 '20 at 14:25

The command [ is equivalent to test command. test doesn't support any kind of advanced matching. test can compare strings with = - comparing strings with == in test is a bash extension.

By doing:

[ "$INFO" == *"x86_64"* ]

You are actually running command like [ "$INFO" == <the list of files that match"x86_64"pattern> ] - the *"x86_64"* undergoes filename expansion. If you would have a file named something_x86_64_something it would be placed there, the same way cat *"x86_64"* would work.

The bash extensions [[ command supports pattern matching. Do:

if [[ "$INFO" == *"x86_64"* ]]

For portable scripting that will always work with any kind of posix shell use case:

case "$INFO" in
*x86_64*) echo yes; ;;
*) echo no; ;;
esac

Cyrus · Answer 2 · 2020-06-17T14:36:39.963

2

With bash version >= 3 you can use a regex:

[[ "$INFO" =~ x86_64 ]]

edited Jun 17 '20 at 14:36

answered Jun 17 '20 at 14:22

Cyrus

84,225
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153

Possible, but unnecessary, because globbing `[[ $INFO == *x86_64* ]] would work as well. – user1934428 Jun 17 '20 at 14:54

score 1 · Answer 3 · answered Jun 17 '20 at 14:42

1

Unsure why it's so but your code starts working after doubling the square brackets:

INFO=$(uname -m && cat /etc/*release)
if [[ "$INFO" = *x86_64* ]]
then
    echo "You are running a 64bit-based system!"
else
    echo "Your system architecture is wrong!"
    exit
fi

Perhaps some explanation can be found under Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash? and alikes.

answered Jun 17 '20 at 14:42

bipll

11,747
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32

1

Sure. Double-brackets ensure that globbing is done on the left-hand side of the operation, i.e. `$INFO`, and not to the files in the directory. BTW, you can omit the double-quotes, if you want to, since no word splitting is done in this situtation. – user1934428 Jun 17 '20 at 14:57

score 0 · Answer 4 · answered Jun 17 '20 at 14:31

0

One way to check 64 bit is to simply grep the output of /bin/arch

if  /bin/arch | grep -q x86_64
then
  echo "it is 64 bit"
else
  echo "it is not"
fi

answered Jun 17 '20 at 14:31

Sam Daniel

1,800
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22

Error when checking for substring in Bash

4 Answers4