Assigning the output of a bash string manipulation to a variable

Question

On Ubuntu 16.04, I have a bash script which I run as root. I want to remove all spaces from the user's input, and assign the trimmed string to a variable.

read -p "Enter username: " USERNAME
echo "$USERNAME" | sed s/' '/''/g
TRIM="$USERNAME" | sed s/' '/''/g
echo $TRIM
echo "Done"

Here's some sample input and output:

Enter username: h j k l
hjkl

Done

$TRIM is empty.
What change do I need to make to get $TRIM to contain the result?

score 5 · Answer 1 · answered Nov 23 '17 at 20:04

5

Use $() to capture the output of a command.

trim=$(echo "$USERNAME" | sed 's/ //g')

However, you don't need a command for this, you can use the bash parameter expansion operator to do substitution.

trim=${username// /}

P.S. Don't use uppercase variable names, they're conventionally reserved for environment variables.

answered Nov 23 '17 at 20:04

Barmar

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If I use the bash parameter expansion operator, I get `./trim.sh: 3: ./trim.sh: Bad substitution` – James Newton Nov 23 '17 at 20:08
Then you're not really using `bash`. See https://stackoverflow.com/questions/8960677/string-replacement-in-bash-bad-substitution-error – Barmar Nov 23 '17 at 20:12
@JamesNewton : You're probably missing the `echo` – sjsam Nov 23 '17 at 20:23
@sjsam There's no `echo` in the command he got the error from. – Barmar Nov 23 '17 at 20:25
@Barmar : The code in question has many problems as you might have noted. For instance, where are quotes to encapsulate the sed command? Might be a copy paste error though. – sjsam Nov 23 '17 at 20:35
1

@sjsam He has the quotes inside: `s/' '/''/g` The second quotes aren't needed, though. – Barmar Nov 23 '17 at 20:37
@Barmar : Oh I see, I overlooked those.. – sjsam Nov 23 '17 at 21:09
Understandable, since it's so unusual. At first I thought he was trying to replace explicit quotes. – Barmar Nov 24 '17 at 06:31

score 1 · Answer 2 · answered Nov 23 '17 at 20:29

There are a couple of things you should note with your script.

First of all, uppercase identifiers like USERNAME are usually reserved for environment variables. So you might well override a shell variable accidentally that causes undesired effects. Same is the case with TRIM.

Secondly, you should check the default shell in your Ubuntu distribution using the SHELL built-in. Do something like

echo $SHELL

If the output is dash or bash, you could use command substitution to achieve your objective.

trim=$(echo "$username" | sed 's/ //g')

Mind that the sed commands themselves should be put under the single quotes.

When you echo trim, use double quotes to prevent word splitting like

echo "$trim" # In your case though the double quotes doesn't make any difference

Actually the quotes make a lot of a difference. If the user inputs an expression containing a shell wildcard character or irregular whitespace, the program without quotes will mess up the output. — tripleee, Nov 24 '17 at 04:24

Assigning the output of a bash string manipulation to a variable

2 Answers2