Shell Cheatsheet

• Commands
  • Chaining commands
• Writing scripts
  • Variables
  • Functions
  • Loops and conditionals
  • Fail checks
• Running scripts
• Resources

Commands

• cat <filename> logs a file

• head -n <number> <filename> logs the first number lines of a file

• tail -n <number> <filename> logs the last number lines of a file

• grep <text> <filename>logs the lines containing text on the file

  • -v --invert-match logs the lines not containing text on the file
  • -i --ignore-case matches independently of the case
  • -E support for all regex instead of <text>
  • -r recursively searchs in all files. Use it with a directory instead of filename

• tree logs the directory structure like a tree, including nested

• sed 's/<oldExpresion>/<newExpresion>/' <filename> logs the file substituting oldExpresion for newExpresion

  • -E support for all regex instead of <oldExpresion>
  • -i 'subfix' modify the original file and save the original one adding subfix to the name (for example, .old). If no subfix is provided, will not create the backup

• mv <filename1> <filename2> renames filename1 to filename2

• > <filename> Will write the shell stdout to the file (replacing content)

• 2> <filename> Will write the shell stderror to the file (replacing content)

• &> <filename> Will write the shell stdout and stderror to the file (replacing content)

• >> <filename> Will append the shell stdout to the file

• echo $? Will print the exit code of the last command. 0 means no error.

Chaining commands

• <command> | <command> Will run the first command and pass its output as the argument for the second command
• <command> ; <command> Will run both commands, independently of their output (i/ errors)

Writing scripts

set -e The script will exit early if any line fails

set -o pipefail The script will mark lines as failed if any piped command on it fails

Variables

Variables are declared just like this varname=value but to be referenced need to have a $ before $varname

Double quotes enable variable interpolation, while single quotes don't. Use with care.

If you don't use quotes at all and your variable has whitespaces, it will be interpreted as different arguments.

myVar=1

echo "This is my var $myVar"
# This is my var 1
echo 'This is my var $myVar'
# This is my var $myVar

To assign the output of a command to a variable, you need to encapsulate it like myVar=$(echo "here").

# This will error
myVar=echo "here"

# This will set myVar to here
myVar=$(echo "here")

Functions

# Declaration
<functionName>() {
  # Code here
  firstArgument=$1
  # etc
}

# Call
<functionName> <args>

# For example

printSomething(){
  echo "$1 and also $2"
}

printSomething "text" "other text"

# will print
# text and also other text

Loops and conditionals

for <varname> in <collection>; do
  # Code here
done

# For example
for file in src/*.md; do
  echo "$file"
done

If is done to work with standard error codes and if a comand exits with anything but a 0, it will evaluate to false.

We can use the -q flag to avoid outputs of anything on the if check

if <command>; then
  # Code here
fi

# For example
if grep -q port "$file"; then
  echo "$file"
fi

Or to compare expresions

if [ <comparison> ]; then
  # Code here
fi

# For example
if [ "$port" -lt 5000 ]; then
  echo "Port is under 5000"
fi

if <condition>; then
  # Code here
else
  # Code here
fi

Some comparators:

• <a> -lt <b> less than
• <a> -gt <b> greater than
• <a> -eq <b> numeric equal to
• -f <filename> filename is a file and exists
• -z "$<variableName>" variableName has a value

Fail checks

You can check the number of arguments provided to a script with $#.

So a fail check can be

if [ $# -eq 0 ]; then
  echo "Arguments missing"
  exit 64
fi

Running scripts

#!/bin/sh at the begining allows the script to be run writing just myscript.sh instead of sh myscript.sh on the command line.

For this to work you may need to set the file as executable (chmod +x <filename>.sh)

You can pass arguments to a script the same way you pass them to a function

# Command line
./my-script.sh arg1 arg2

# Script

echo "$1 - $2"

# will print
# arg1 - arg2

Resources

• https://learnxinyminutes.com/docs/bash/