Bash – Why does –text=“$@” only pass the first word

bashquotingshell

When I run the following script with some arguments like arg1 arg2 arg3:

#!/bin/bash
zenity --entry --text="$@"

zenity creates an entry dialog with this text: "arg1" whereas I expect "arg1 arg2 arg3"

If I use a variable like the following script it shows all arguments for the entry text.

#!/bin/bash
text="$@"
zenity --entry --text="$text"

What's the difference between these scripts? Why does the first one replace $@ with the first argument only?

Best Answer

$@ expands to separate words (whereas $* expands to a single word), as explained in the bash manual. Thus, when you write

zenity --text="$@"

it expands to

zenity --text="$1" "$2" "$3"

However, shell variable assignments do not undergo word splitting. Note that field / word splitting is omitted in the list of expansions for variable assignments in the bash manual. This behavior is consistent with the POSIX spec. So, when you write

text="$@"

the variable text gets all of the positional parameters as a single word, equivalent to if you had written text="$*". Indeed, this is the reason double quotes are often unnecessary in variable assignments. Both

text=$@

and

text=$*

are perfectly safe.

So,

text=$@
zenity --option="$text"

expands "$text" to a single word, which is why this works. Note that the --option="$@" is just a normal argument to the command zenity, and not a shell variable assignment, which is why word splitting takes place here but not in text=$@.

Related Solutions

Bash – Capture in shell variables the arguments after a command (between brackets)

I'd like to find a regular pattern to include in a shell script so that n variables will contain the n arguments

The following creates a shell array arglist that contains each of the arguments:

$ readarray -t arglist < <(echo "\command{arg1,
    arg2 ,
    arg3
    }" | sed -n '/\\command/{ :a;/}/!{N;b a}; s/\\command{//; s/[ \n}]//g; s/,/\n/g; p}')

By using the declare statement, we can see that it worked:

$ declare -p arglist
declare -a arglist='([0]="arg1" [1]="arg2" [2]="arg3")'

Here is another example with the arguments on one line:

$ readarray -t arglist < <(echo "\command{arg1, arg2, arg3, arg4}"  | sed -n '/\\command/{ :a;/}/!{N;b a}; s/\\command{//; s/[ \n}]//g; s/,/\n/g; p}')

Again, it works:

$ declare -p arglist
declare -a arglist='([0]="arg1" [1]="arg2" [2]="arg3" [3]="arg4")'

Note that the space in < <( is essential. We are redirecting input from a process substitution. Without the space, bash will try something else entirely.

How it works

The sed command is a bit subtle. Let's look at it a piece at a time:

-n

Don't print lines unless explicitly asked.
/\\command/{...}

If we find a line that contains \command, then perform the commands found in the braces which are as follows:
:a;/}/!{N;b a}

This reads lines into the pattern buffer until we find a line that contains }. This way, we get the whole command in at once.
s/\\command{//

Remove the \command{ string.
s/[ \n}]//g

Remove all spaces, closing braces, and newlines.
s/,/\n/g

Replace commas with newlines. When this is done, each argument is on a separate line which is what readarray wants.
p

Print.

Bash – Fixing getopts Function Running Only Once

bash getopts use an environment variable OPTIND to keep track the last option argument processed. The fact that OPTIND was not automatically reset each time you called getopts in the same shell session, only when the shell was invoked. So from second time you called getopts with the same arguments in the same session, OPTIND wasn't changed, getopts thought it had done the job and do nothing.

You can reset OPTIND manually to make it work:

$ OPTIND=1
$ f -a 123
-a was triggered, Parameter: 123

or just put the function into a script and call the script multiple times.

zsh getopts is slightly different. OPTIND was normally reset to 1 each time upon exit from shell function.

Best Answer

Related Solutions

Bash – Capture in shell variables the arguments after a command (between brackets)

How it works

Bash – Fixing getopts Function Running Only Once

Related Question