It's always amazing observing experts. It often leaves you inspired, motivated and hungry for more professional growth of your own. Recently, I switched teams and now work alongside someone who has been writing bash scripts professionally for longer than I have been alive. Watching him approach problems has left me in awe.
This week, I learnt about built-in bash parameters from him and thought to explore them a little more.
NB: The shell treats these parameters specially. They can only be referenced. Assignment to them is not allowed.
Parameters explored
1. Asterisk *
| Variable | Usage |
|---|---|
| $* | Stores all the arguments that were entered on the command line ($1 $2 ...) |
For example take script foo.sh below
#!/bin/bash
# loops over arguments and prints them in a new line
for i in $* ; do
echo -e "$i\n"
done
Now run the script and pass a few arguments
$ ./foo.sh arg-1 arg-2 arg-3 arg-4
arg-1
arg-2
arg-3
arg-4
# notice double quotes around "arg-2 arg-3" are ignored
# they are treated as 2 seperate arguments
$ ./foo.sh arg-1 "arg-2 arg-3" arg-4
arg-1
arg-2
arg-3
arg-4
However, when we place $* in double quotes "$*" all the arguments are taken as a single string
#!/bin/bash
#notice the change with "$*"
for i in "$*" ; do
echo -e "$i\n"
done
$ ./foo.sh arg-1 arg-2 arg-3 arg-4
arg-1 arg-2 arg-3 arg-4
2. At sign @
| Variable | Usage |
|---|---|
| $@ | Stores all the arguments that were entered on the command line, individually quoted ("$1" "$2" ...) |
$@ is similar to $* but with a subtle difference.
#!/bin/bash
for i in $@ ; do
echo -e "$i\n"
done
$ ./foo.sh arg-1 arg-2 arg-3 arg-4
arg-1
arg-2
arg-3
arg-4
# same behaviour as $*
# double quotes are ignored
$ ./foo.sh arg-1 "arg-2 arg-3" arg-4
arg-1
arg-2
arg-3
arg-4
However, $@ behaviour changes when in double quotes "$@"
#!/bin/bash
#notice the change with "$@"
for i in "$@" ; do
echo -e "$i\n"
done
# double quotes are treated as a single argument
$ ./foo.sh arg-1 "arg-2 arg-3" arg-4
arg-1
arg-2 arg-3
arg-4
With "$@", all the arguments that were entered are treated individually quoted ("$1" "$2"...)
This is generally the behaviour we often want when writing scripts and how it differs from $*
3. Hashtag #
| Variable | Usage |
|---|---|
| $# | Stores the number of command-line arguments that were passed to the shell program. |
Take foo.sh
#!/bin/bash
echo "Number of arguments entered is : $# "
$ ./foo.sh arg-1 arg-2 arg-3 arg-4
Number of arguments entered is : 4
$ ./foo.sh arg-1 arg-2
Number of arguments entered is : 2
4. Question mark ?
| Variable | Usage |
|---|---|
| $? | Stores the exit value of the last command that was executed. |
It's also called exit status and this variable stores the exit value of the last command that was executed. It is an integer number.
If $? equals 0, the previously run command was successful.
If $? is between 1-255, the command failed.
If $? equals 127, then the command is not found.
Example usage :
$ echo "Hello there!" > file.txt
$ cat file.txt
Hello there!
$ echo $?
0
# returns 0 as the last run command was successful.
$ cat non-existent-file.txt
cat: non-existent-file.txt: No such file or directory
$ echo $?
1
# returns 1 as the last run command was unsuccessful.
You can see the usefullness of $? in this urlchecker script bar.sh
#!/bin/bash
curl --output /dev/null --silent ${1} && exit_code=$? || exit_code=$?
if [ ${exit_code} -eq 0 ]; then
echo "${1} is a valid url"
else
echo "Sorry, ${1} is NOT a valid url"
fi
Invoking the script and passing in both valid and invalid urls
$ ./bar.sh https://www.google.com
https://www.google.com is a valid url
$ ./bar.sh https://www.not-a-valid-website.com
Sorry, https://www.not-a-valid-website.com is NOT a valid url
5. Zero 0
| Variable | Usage |
|---|---|
| $0 | Stores the name of the script that is currently being executed. |
Take script bar.sh
#!/bin/bash
echo "This is the name of this shell script : $0"
$ ./bar.sh
This is the name of this shell script : ./bar.sh
Just running in the terminal, you get :
$ echo $0
-bash
In essense, $0 holds the name of the currently executing script in bash.
You can see more on special bash parameters from the Bash reference manual
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