Command substitution with strings in Bash [duplicate] - linux

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Why does adding spaces around bash comparison operator change the result?
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I want to check which lines of the file /etc/passwd end with the "/bin/bash" string (field number 7, ":" as delimiter).
So far, I've written the following code:
while read line
do
if [ $("$line" | cut -d : -f 7)=="/bin/bash" ]
then
echo $line | cut -d : -f 1
echo "\n"
fi
done < /etc/passwd
Currently, executing the script throws errors that show a bad interpretation (most likely due to the syntax).
I'd appreciate if you could help me.

Answer 1

You MUST surround the == operator with spaces. [ and [[ do different things based on how many arguments are given:
if [ "$( echo "$line" | cut -d: -f7 )" == "/bin/bash" ]; ...
I would actually do this: parse the line into fields while you're reading it.
while IFS=: read -ra fields; do
[[ ${fields[-1]} == "/bin/bash" ]] && printf "%s\n\n" "${fields[0]}"
done < /etc/passwd

Answer 2

This line is wrong:
if [ $("$line" | cut -d : -f 7)=="/bin/bash" ]
Also, this is not going to do what you want:
echo "\n"
Bash echo doesn't understand backslash-escaped characters without
-e. If you want to print a new line use just echo but notice that
the previous echo:
echo $line | cut -d : -f 1
will add a newline already.
You should always check your scripts with
shellcheck. The correct script would be:
#!/usr/bin/env bash
while read -r line
do
if [ "$(echo "$line" | cut -d : -f 7)" == "/bin/bash" ]
then
echo "$line" | cut -d : -f 1
fi
done < /etc/passwd
But notice that you don't really need a loop which is very slow and
could use the following awk one-liner:
awk -v FS=: '$7 == "/bin/bash" {print $1}' /etc/passwd

Answer 3

Instead of looping through the rows, and then checking for the /bin/bash part, why not use something like grep to get all the desired rows, like so:
grep ':/bin/bash$' /etc/passwd
Optionality, you can loop over the rows by using a simple while;
grep ':/bin/bash$' /etc/passwd | while read -r line ; do
echo "Processing $line"
done

Answer 4

Don't do while read | cut. Use IFS as:
#!/bin/sh
while IFS=: read name passwd uid gid gecos home shell; do
if test "$shell" = /bin/bash; then
echo "$name"
fi
done < /etc/passwd
But for this particular use case, it's probably better to do:
awk '$7 == "/bin/bash"{print $1}' FS=: /etc/passwd
The issue your code has is a common error. Consider the line:
if [ $("$line" | cut -d : -f 7)=="/bin/bash" ]
Assume you have a value in $line in which the final field is /bin/dash. The process substitution will insert the string /bin/dash, and bash will attempt to execute:
if [ /bin/dash==/bin/bash ]
since /bin/bash==/bin/bash is a non-empty string, the command [ /bin/bash==/bin/bash ] returns succesfully. It does not perform any sort of string comparison. In order for [ to do a string comparison, you need to pass it 4 arguments. For example, [ /bin/dash = /bin/bash ] would fail. Note the 4 arguments to that call are /bin/dash, =, /bin/bash, and ]. [ is an incredibly bizarre command that requires its final argument to be ]. I strongly recommend never using it, and replacing it instead with its cousin test (very closely related, indeed both test and [ used to be linked to the same executable) which behaves exactly the same but does not require its final argument to be ].