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Useful bash commands

Command Description
\u Add the name of the current user when used in the prompt
\w Add the full path of the current directory, starting from ~ when used in the prompt
\W Add just the name of the current directory when used in the prompt
alias List bash aliases in current session
zip -r archive_name.zip folder_to_compress Zip folder_to_compress into archive_name.zip
unzip archive_name.zip Unzip zipped archive archive_name.zip
array=( item1 item2 item3 ) Create an array
"${array[@]}" Expand items in an array
lsof -i :port Display process on port
kill -9 PID Kill process PID
$OSTYPE OS type
$USERNAME Logged-in username
$HOME Home directory path
$BASH_VERSION Shell version
[[ -d directory ]] Test a directory exists
[[ -f file ]] Test a file exists
[[ -e file_or_directory ]] Test for existence of file or directory
fc Open the previous command in the default terminal editor, then run it on save and close
<space> <command> Don't store command in history

Colours in bash: guide

Redirection

Command Description
command>file Redirect output of command to file, overwriting it if it already exists
command>>file Redirect and append output of command to file
file<command Redirect contents of file into stdin of command

File descriptors (FD)

  • 0 - stdin
  • 1 - stdout
  • 2 - stderr
Command Description
&>file Redirect both standard output and standard error to file
i>/dev/null Redirect FD i to null device
i>&j Redirect FD i's stream to FD j's current stream
i<&- Close input FD i
i>&- Close output FD i

Exit codes

Use $? to get the exit code of last process.

  • 1 - Catchall for general errors
  • 2 - Misuse of shell builtins (according to Bash documentation)
  • 126 - Command invoked cannot execute
  • 127 - “command not found”
  • 128 - Invalid argument to exit
  • 128+n - Fatal error signal “n”
  • 130 - Script terminated by Control-C
  • 255\* - Exit status out of range

Parameters/variables

  • Assignment and reference: assign with a=value and refer with $a - if value is a string with spaces in it, it must be in single or double quotes; double quotes are required if any variables are included
  • Command substitution: $(command) e.g. $(cat hello.txt) - run a subshell to get the output of the command and substitute it in
  • Referring to positional variables: $i or ${j} - either are acceptable but where i >= 10, variables must be in the latter format

Note: bash variables can nearly always be put in double quotes ("") with no adverse effects, and can help avoid catastrophic mistakes due to (perhaps unknown) spaces in variable values (such as accidentally deleting all files in the home directory).

Command Description
${#variable} Get the length of variable
$# Return number of positional arguments
$* Return all positional arguments

Truth values

Expression/type Truth evaluation Detail
true True Truth command
false False Falseness command
Empty string False
Non-empty string True
Positive or negative number, including 0 True
Uninitialised variable False
"true" True Just a string
"false" True Just a string
$true False Uninitialised variable
$false False Uninitialised variable

Comparisons

If [[ ]] are used for comparing rather than [ ], all the following comparisons can be used:

Comparator Intuitive notation Less intuitive notation Operand type
Less than < -lt (numeric only) String, numeric
Less than or equal -le (numeric only) Numeric
Greater than > -gt (numeric only) String, numeric
Greater than or equal -ge (numeric only) Numeric
Equal == -eq (numeric only) String, numeric
Not equal != -ne (numeric only) String, numeric
Null -z String, numeric
Not null -n String, numeric

Conditionals

if

if [[ condition ]]; then
    action
elif [[ another_condition ]]; then
    another_action
else
    something_else
fi

Note: conditionals terminate with exit code 0 if true.

case

case expression in
    pattern1 )
        statements ;;
    pattern2 )
        statements ;;
    ...
esac

Loops

for

Example 1

for i in 1 2 3 4
    do
        actions
    done

Example 2

for i in {start..stop}
    do
        actions
    done

Example 3

for ((i=start; i<=stop; i++))
    do
        actions
    done

while

while [[ condition ]]
do
    actions
done

Note: if e.g. while true is used, the loop will continue until CTRL+C is executed by the user.

Functions

my_function() {
    echo "Get positional arguments to function by using $1, $2 etc."
}

Function return values can be accessed and assigned to a variable via command substitution:

output=$( my_function arg1 arg2 ... )

HereDocs

command << delimiter
line1
line2
line3
delimiter

Ignore tabs by using -, e.g.:

cat <<- EOF
    line1
    line2
    line3
EOF

Send to a file:

cat > myfile <<- xxx
    Content
    More content
xxx