Lesson 15 · Networking, scripting and internals

Bash scripting

A Bash script is a text file of commands that Bash runs top to bottom, so a job you type every day can run itself. Follow one seven-line script, count.sh, as it runs: the kernel reads the shebang and starts bash, a variable is made, a glob turns *.txt into a list of file names, a for loop walks that list while arithmetic counts, echo prints each result, and the script ends with an exit status.

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Fig 15 · Interactive

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5 quick questions

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01What does the first line #!/bin/bash do?

When you run ./count.sh, the kernel reads the #! line and starts the interpreter it names, /bin/bash, with the script's path. To bash itself the line is just a comment. Making the file executable is chmod +x's job.

02Which line correctly sets a variable named count to 0?

No spaces around the =. With spaces, Bash reads count as the name of a command to run with the arguments = and 0, which fails.

03A folder holds hello.py, notes.txt and todo.txt. What does for f in *.txt loop over?

Bash expands the glob *.txt to the names that end in .txt, sorted: notes.txt then todo.txt. hello.py does not match. If nothing matched, the loop would see the text *.txt itself (unless nullglob is on).

04count holds 1. What does count=$((count + 1)) leave in count?

$(( )) is arithmetic expansion: Bash works out 1 + 1 and puts 2 in its place, so the line becomes count=2.

05Right after a script finishes, echo $? prints 0. What does that mean?

$? holds the exit status of the last command. 0 means success and any other number means failure. A script's status is the status of the last command it ran, unless it calls exit with a number.

Key terms

Glossary

Script: A text file of commands that a program such as Bash runs from top to bottom.
Shebang: The first line of a script, #! followed by a path such as /bin/bash. It tells the kernel which program runs the file.
chmod +x: A command that gives a file execute permission, so you can run it as ./name.
Variable: A name that holds a value inside the running bash, made with name=value and read with $name.
Glob: A pattern with wildcards, such as *.txt. Bash replaces it with the sorted list of file names that match.
for loop: for name in list; do ... done runs the lines between do and done once for each item, with name set to that item.
Arithmetic expansion: $(( expression )): Bash works out the sum (whole numbers only) and puts the answer in its place.
Exit status: A number a command returns when it ends: 0 for success, 1 to 255 for failure. $? holds the last one.

Fine print

Notes and sources

Some scripts start with #!/usr/bin/env bash instead, which finds bash wherever it is installed. You can also run a script without chmod +x by typing bash count.sh.
Bash sorts the names a glob expands to using your locale's order. If no file matches, the pattern stays as it is (the loop would see the text *.txt) unless you turn on shopt -s nullglob.
$(( )) works with whole numbers only. Bash variables are text by default; inside $(( )) Bash reads them as numbers.
A script's exit status is the status of the last command it ran, here echo, which returned 0. Use exit 1 to report a failure yourself.

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