I ran into a command syntax issue for a conditional statement in a script that someone else wrote. The script is as follows (truncated).

#! /bin/bash

# app_upgrade.sh

# Verify file integrity
filehash=$( md5sum install_package.tgz )
md5hash=$( cat install_package.tgz.md5 )

if [ -z $( diff <(echo $md5hash) <(echo $filehash) ) ]; then
    printf "File Integrity Check Passed"
else
    printf "File Integrity Check Failed"
    exit
fi

When I run this script, it fails when trying to interpret the conditional statement due to an unexpected opening parenthesis. The exact error reported to the CLI is as follows.

app_upgrade.sh: command substitution: line 118: syntax error near unexpected token `('
app_upgrade.sh: command substitution: line 118: ` diff <(echo $md5hash) <(echo $filehash) )'

I verified that diff is an executable command on my system by the same user as that running the script. I also ran the diff <(echo $md5hash) <(echo $filehash) from the CLI and this worked without any issues. I also tried to escape the parentheses but that also failed. I am stumped as to why this is causing an issue.

As a workaround, I tried a few other conditionals since I would not have used diff for the comparison had I been writing the script in the first place. I tried the following replacements for the conditional specified in the script above.

if [ "$filehash" = "$md5hash" ] However, this did not work. Even though the hashes were the same, the conditional caused the comparison to fail unexpectedly.

if [[ "$filehash" == "$md5hash" ]] This finally worked.

In summary, my questions are:

1. Why did the script fail with a syntax error when trying to interpret the $( diff <(echo $md5hash) <(echo $filehash) within the original conditional statement?

2. In my updated conditional statements, assuming both hashes are the same, why did if [ "$filehash" = "$md5hash" ] fail but if [[ "$filehash" == "$md5hash" ]] succeed? From my research, it looks like both are valid ways of comparing strings in bash.

Thanks in advance!

CodePudding user response：

When you run your script against shellcheck linter you can see the error

In ./test.sh line 9:
if [ -z $( diff <(echo $md5hash) <(echo $filehash) ) ]; then
        ^-- SC2046 (warning): Quote this to prevent word splitting.
                       ^------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                        ^-------^ SC2086 (info): Double quote to prevent globbing and word splitting.

You have to quote the if expression to avoid word splitting:

#! /bin/bash

# app_upgrade.sh

# Verify file integrity
filehash=$( md5sum install_package.tgz )
md5hash=$( cat install_package.tgz.md5 )

# mind the `"` quotation marks around 
if [ -z "$( diff <(echo $md5hash) <(echo $filehash) )" ]; then
    printf "File Integrity Check Passed"
else
    printf "File Integrity Check Failed"
    exit
fi

CodePudding user response：

My issue was shell related. The root cause of my issue was a noexec security constraint in the /home/ directory so I could not run ./app_upgrade.sh from there. However, sh app_upgrade.sh worked fine and executed the script, but I was not running it as bash as I intended. Using bash app_upgrade.sh allowed the script to run properly. So the root cause here was a permissions issue in the directory I was executing the script from causing me to incorrectly execute the bash script with the sh command.

In summary:

[user@host ~]$ ./app_upgrade.sh resulted in bash: ./app_upgrade.sh: Permission denied

[user@host ~]$ sh app_upgrade.sh executed the script but not as bash so this caused an issue interpreting one of the lines of code properly.

[user@host ~]$ bash app_upgrade.sh executed the script properly without errors.

Had I originally run this in a directory that did not have noexec constraint then the issue would never have manifested.

Thanks for the all the comments, suggestions and help!