Bash Commands for Files, Directories and Variables

What it is

Bash is the command line shell found on most Linux systems and many developer machines, and it is the usual way to run commands and scripts.

Developers and network automation engineers use it to move around the file system, manage files, set variables and chain tools together.

This page covers the basic commands that appear in exam questions and scripts.

How it works

You give Bash a command, with options and arguments, and it prints the result or acts on files.

Directory navigation

$ pwd
/home/user
$ ls
$ ls -l
$ ls -a
$ cd projects
$ cd ..
$ cd ~
$ cd /etc

A path that starts with a slash is absolute, and it begins at the root of the file system. Any other path is relative to the current directory. A single dot means the current directory.

File and directory management

$ mkdir backups
$ mkdir -p app/logs
$ touch notes.txt
$ cp notes.txt backups/
$ mv notes.txt notes_old.txt
$ rm notes_old.txt
$ rm -r backups

There is no recycle bin. A removed file is gone, so check the name before you press Enter.

Viewing and searching text

$ cat config.yaml
$ head -n 5 app.log
$ tail -n 20 app.log
$ grep ERROR app.log
$ wc -l app.log

cat prints a file. head and tail show the first and last lines. grep prints lines that match a pattern, and wc -l counts lines.

Pipes and redirection

The pipe sends the output of one command into the next. Redirection sends output to a file.

$ grep ERROR app.log | wc -l
$ ls -l > files.txt
$ echo "done" >> files.txt

A single > creates the file or overwrites it. A double >> appends to the end.

Permissions

$ ls -l script.sh
-rw-r--r-- 1 user user 120 Jan 10 09:00 script.sh
$ chmod +x script.sh
$ ./script.sh

The first column shows the type and then three groups of three letters, for the owner, the group and everyone else. The letters are r for read, w for write and x for execute. The chmod +x command adds execute permission, so you can run the file with ./script.sh.

Environment variables

An environment variable is a named value that programs can read. Set one with export, read it with a dollar sign, and list all with env.

$ export API_BASE=sandbox.example
$ echo $API_BASE
sandbox.example
$ env
$ echo $PATH
$ unset API_BASE

A plain assignment such as NAME=value sets a shell variable only. Adding export passes it to programs that you start.

Variables set this way last until the shell closes. To keep them, add the line to a startup file such as .bashrc.

PATH is a list of directories where Bash looks for commands. HOME is your home directory.

Scripts read variables with the same dollar sign, and Python reads them with os.environ. This is the usual way to pass a token to a script without writing it in the code.

$ export API_KEY=abc123
$ python3 script.py

Getting help

man ls opens the manual page for a command, and many tools accept --help.