Bash cheatsheet

Variables

Define variable

name="John"

Use the variable

echo $name
echo "$name"
echo "${name}!"

Substitution/slicing

• echo "${name/J/j}" “john” (substitution)
• echo "${name:0:2}" “Jo” (slicing)
• echo "${name::2}" “Jo” (slicing)
• echo "${name::-1}" “Joh” (slicing)
• echo "${name:(-1)}" “n” (slicing from right)
• echo "${name:(-2):1}" “h” (slicing from right)

Change case

• echo "${str,}" lowercase 1st letter
• echo "${str,,}" all lowercase
• echo "${str^}" uppercase 1st letter
• echo "${str^^}" all uppercase

Default values

• ${foo:-val} $foo, or val if unset (or null)
• ${foo:=val} Set $foo to val if unset (or null)
• ${foo:+val} val if $foo is set (and not null)
• ${foo:?message} Show error message and exit if $foo is unset (or null)

Shell execution

echo "Current directory $(pwd)"
echo "Current directory `pwd`" 

Single line

# Single line comment

Multi line

: '
This is a
multi line
comment
'

• $? Exit status of last task
• $! PID of last background task
• $$ PID of shell
• $0 Filename of the shell script
• $_ Last argument of the previous command

For loop

for i in {1..10}; do
  echo $i;
done

for i in $(seq 1 10); do
  echo $i;
done

C-like version

for ((i = 0 ; i < 100 ; i++)); do
  echo "$i"
done

---

While

while read -r line; do
  echo "$line"
done <file.txt

Infinite loop

while true; do
  ···
done

Define function

function hello {
    echo "hello world"
}

hello() {
    echo "hello world"
}

Arguments

• $# Number of arguments
• $* All arguments (as a single word)
• $@ All arguments (as separate strings)
• $1, $2, $3 First, second, third argument
• $_ Last argument of the previous command

File

• [[ -e FILE ]] Exists
• [[ -r FILE ]] Readable
• [[ -h FILE ]] Symlink
• [[ -d FILE ]] Directory
• [[ -w FILE ]] Writable
• [[ -s FILE ]] Size is > 0 bytes
• [[ -f FILE ]] File
• [[ -x FILE ]] Executable
• [[ FILE1 -ef FILE2 ]] Same files

Example

if [[ -e "file.txt" ]]; then
  echo "File exists"
fi

---

String

• [[ -z STRING ]] Empty
• [[ -n STRING ]] Not empty
• [[ STRING1 == STRING2 ]] Equal
• [[ STRING1 != STRING2 ]] Not Equal

Example

if [[ -z "$string" ]]; then
  echo "String is empty"
else
  echo "String is not empty"
fi

Number

• [[ NUM1 -eq NUM2 ]] Equal
• [[ NUM1 -ne NUM2 ]] Not equal
• [[ NUM1 -lt NUM2 ]] Less than
• [[ NUM1 -le NUM2 ]] Less than or equal
• [[ NUM1 -gt NUM2 ]] Greater than
• [[ NUM1 -ge NUM2 ]] Greater than or equal

Example

if [[ $n -gt $n2  ]]; then
  ...
elif [[ $n -eq $n2 ]]; then
  ...
else
  ...
fi

---

Logical operators

• [[ ! EXPR ]] Not
• [[ X && Y ]] And
• [[ X || Y ]] Or

if [[ $x && $y ]]; then
  ...
fi

case "$1" in
  start)
    echo "start"
    ;;
  stop)
    echo "stop"
    ;;
  *)
    echo "Usage: $0 {start|stop}"
    ;;
esac

• ./program > output.txt stdout to file
• ./program >> output.txt append stdout to file
• ./program 2> error.log stderr to file
• ./program 2>&1 stderr to stdout
• ./program 2>/dev/null stderr to /dev/null
• ./program >output.txt 2>&1 stdout and stderr to file, equivalent to &>
• ./program &>/dev/null stdout and stderr to /dev/null
• ./program < input.txt stdin from input.txt
• ./program | grep "file" stdout to another programs’ stdin

cat <<END
...
END

Show history

history

Clear history

history -c

---

Expansions

• !$ Expand last parameter of most recent command
• !* Expand all parameters of most recent command
• !-n Expand nth most recent command
• !n Expand nth command in history
• !<command> Expand most recent invocation of command <command>

---

Temporarily disable bash history

Disable history file until end of session:

unset HISTFILE

Disable/enable using set in one session:

set +o history
# commands here won't be saved
set -o history

Set environment variables:

export HISTSIZE=0
export SAVEHIST=0

---

Show Date and Time

Display time and date before each command in history

HISTTIMEFORMAT="%d-%m-%y %r " history

• %d – Day
• %m – Month
• %y – Year
• %T / %r – Time in 24/12 hours format

To set this permanently, add HISTTIMEFORMAT variable definition to your .bashrc file

export HISTTIMEFORMAT="%d-%m-%y %r "