Command Basics ¶

Commands usually have an option "help". ⛪

$ man -h
$ find -help
$ gcc --help

⚠️ There is no echo/feedback when writing a password.

When using commands, you will most likely use, or have to use strings. Most of the time, quotes are optional, and command arguments do not need to be quoted 🎯.

$ echo Hello, World
$ echo 'Hello, World'
$ echo "Hello, World"

Concatenation is also implicit. All of these are the same:

$ cat abc
$ cat "a"bc
$ cat "ab"c
$ cat "abc"
$ cat "a""bc"

There are times when we must quote an argument 🎣. For instance, when there is a space inside a name/path.

$ ls /Documents/my bookmarks/ # ❌
$ ls "/Documents/my bookmarks/" # ✅
$ ls '/Documents/my bookmarks/' # ✅
$ ls /Documents/my\ bookmarks/ # ✅

👉 The main difference between single and double quote is that in the former, variable are NOT interpreted. Refer to the scripts section.

To calculate the length of a string:

$ echo -n "xxx" | wc -c  # use wc
$ v="xxx"; echo ${#v}    # use variable length 

$ expr length "Sarah"
5

$ expr substr "Sarah" 2 3
ara

$ expr index "Sarah" S
1
$ expr index "Sarah" w
0

You may want to do some stuff involving calculations. The shell won't interpret any calculations, so you must use $[...] or $((...)) to evaluate something.

$ four=$[3+1]
$ four=$((3+1)) # same

Example of increasing a variable by one (loops are covered in scripts).

i=0
while [ $i -lt 5 ]; do
   i=$[i+1] 
done

It's possible for a command output, to be used as the next command input using a pipe: their_output | is_my_input.

Example 🔥:

The command wc -l is used to count the number of lines in a text. The text could be a file, or some input like below:

$ wc -l
aaa
bbb
2 # two lines

➡️ As a reminder, use CTRL+D to indicate the end of the input.

The command find . -name toto find recursively every file named "toto" starting from the folder '.'. Its output is something like:

$ find . -name toto
./mem/all_toto_copy/toto
./mem/all_toto/toto
[...]
./mem/toto

Using a pipe, we can send the output of find to wc, and find the number of files (lines) returned by find:

$ find . -name toto | wc -l
57 # find returned 57 lines

There are three reasons to chain commands:

• ✅ execute a command if the previous one was successful (&&)
• ❌ execute any command until one is successful (||)
• 🤝 put many commands in one line/statement (;)

For the two first ones, a command is said to be successful is the process exit code is zero. Otherwise, it failed.

$ cat not_found # a non-successful command
$ echo $?
1 # failure

Examples 🔥:

Execute two commands in one go

$ ls toto; ls tata

Execute the second ls if the first was successful.

$ ls toto && ls tata

Execute the second ls if the first failed.

$ ls toto || ls tata

Redirections allow us to:

• 🛁 fill stdin with a source such as a file
• 🗃️ redirect stdout to a file
• 🚩 redirect stderr to a file

There are multiple and varied uses:

• 🚀 fill commands needing user input with data from a file
• 🧑‍🏭 store results that we took a long time to compute, to avoid losing them or having to compute them again
• 🧯 redirect error to /dev/null (discard errors)
• 🪲 store errors for debugging purposes
• ...

Examples 🔥:

Redirect stdout to a file (save result)

$ echo "Hello, World" > myFile

Append stdout to a file

$ echo "Hello, World" >> myFile

Use a file instead of user input

$ tee < original.txt
Hello, World

Redirect errors to a file/discard them

$ cat not_found 2> onl_errors_will_be_stored_here
$ cat not_found 2> /dev/null # discard them

Redirect stdout tostderr (uncommon usage)

$ >&2 echo "xxx"
$ echo "xxx" >&2

Merge stderr with stdout (uncommon usage)

$ cat not_found 2>&1

Discard stdout, but keep stderr (uncommon usage)

$ echo xxx &> /dev/null

Redirect both stdout andstderr (uncommon usage)

$ echo xxx > output.txt 2>&1 # ⚠️ Order is important

Command substitution ¶

Command substitution is mainly used in scripts, to store the result of a command inside a variable. In a nutshell, a command output is used as a parameter of another command: xxx $(yyy) or xxx `yyy`.

Example 🔥: all of these commands are the same

$ ls ~
$ ls `echo ~`
$ ls $(echo ~)

Subshell ¶

A subshell is a shell that is launched within another shell, allowing us to execute commands without changing the parent shell environment.

Example 🔥: In the example below, we create a variable in a subshell, and print it. We also try to print the variable outside the subshell, but since the variable does not exist, the output is empty.

$ (cat=toto; echo "1. $cat") && echo "2. $cat"
1. toto
2.

👉 It could be used to temporarily change location, set an environment variable for one command...

• terminal shortcuts (CTRL +/-/L/D, scroll button...)
• CTRL+SHIFT+T to open a terminal.