A shell script is a text file of commands, run top to bottom as if you'd typed them. The first line, the shebang, says which program runs the file. For Bash it's #!/bin/bash. Lines starting with # after that are comments.
#!/bin/bash
# hello.sh: greet someone
echo "Hello, $1!"
echo "You gave me $# arguments."Save it, give it execute permission once with chmod +x hello.sh, and run it with its path:
$ chmod +x hello.sh
$ ./hello.sh Dana
Hello, Dana!
You gave me 1 arguments.Inside a script, $1, $2 and so on are the arguments, $# is how many there are, $0 is the script's own name and "$@" is all of them, each kept whole.
Every command ends with an exit status: 0 means success and anything else means some kind of failure. $? holds the last one. Scripts end with exit 0 (fine) or exit 1 (something went wrong) so whatever ran them can tell.
Two lines that make scripts safer: set -e stops the script at the first command that fails, instead of carrying on and making things worse, and set -u treats a misspelled variable as an error instead of quietly using an empty value.
What is the first line of a Bash script?
| #!/bin/bash | The shebang: run this file with Bash. |
|---|---|
| chmod +x file; ./file | Make it runnable; run it. |
| $1 $2 … $# "$@" $0 | Arguments, how many, all of them, the script's name. |
| $? | The last command's exit status (0 is success). |
| exit N | End the script with status N. |
| set -e, set -u | Stop on failure; error on unset variables. |