Options

Published

2024-08-28

Caution

This section is being revised. Thank you for your patience.

Unix command options (sometimes called flags) are often preceded by a hyphen (-) or two (-- for long-form options) and modify the behavior of a command. Options provide flexibility, control, and, in some cases, enable a vast array of additional functionalities.

Option types

Arguments can be short or long:

Short options are typically a single dash followed by a single letter (e.g., -l) and they modify the command behavior in a specific, often concise way.

Long options usually use two dashes followed by a word or compound words (e.g., --long-listing) and provide a more descriptive way to specify options, making scripts and commands more readable.

Short options

Example

ls -l data lists files in data in a long format, showing detailed information like permissions, owner, size, and modification date:

ls -l data
# total 112
# -rw-r--r--@  1 mjfrigaard  staff   8798 May 15 21:59 README.md
# -rw-r--r--@  1 mjfrigaard  staff   6122 Apr 10 14:04 music_vids.tsv
# -rw-r--r--@  1 mjfrigaard  staff  12057 Apr 22 14:11 pwrds.tsv
# drwxr-xr-x@ 10 mjfrigaard  staff    320 May 15 21:55 raw
# -rw-r--r--@  1 mjfrigaard  staff   4417 Apr 10 14:01 trees.tsv
# -rw-r--r--   1 mjfrigaard  staff   4814 Apr 10 14:07 vg_hof.tsv
# -rw-r--r--@  1 mjfrigaard  staff    462 Apr 15 14:07 wu_tang.psv
# -rw-r--r--@  1 mjfrigaard  staff    263 Apr 10 09:39 wu_tang.tsv

Long options

Example 1

tree data --sort size lists files in order according to size:

tree data --sort size
# ├── ajperlis_epigrams.txt
# ├── pwrds.csv
# ├── pwrds.tsv
# ├── README.md
# ├── music_vids.tsv
# ├── vg_hof.tsv
# ├── trees.tsv
# ├── roxanne.txt
# ├── wu_tang.dat
# ├── who_tb_data.tsv
# ├── who_tb_data.txt
# ├── wu_tang.txt
# ├── wu_tang.csv
# ├── wu_tang.tsv
# ├── roxanne_rev.txt
# └── roxanne_orig.txt
# 
# 1 directory, 16 files

Example 2

grep --help displays usage information for the grep command, helping users understand available options and syntax.

grep --help
#> usage: grep [-abcdDEFGHhIiJLlMmnOopqRSsUVvwXxZz] [-A num] [-B num] [-C[num]]
#>  [-e pattern] [-f file] [--binary-files=value] [--color=when]
#>  [--context[=num]] [--directories=action] [--label] [--line-buffered]
#>  [--null] [pattern] [file ...]

Example 3

The --version option is commonly used to get version information for various commands.

grep --version  
# grep (BSD grep, GNU compatible) 2.6.0-FreeBSD

Note that not all commands support the long-form option syntax.

Modifying behavior

Some options modify the commands behavior.

Example

cp-n source.txt dest.txt does not overwrite the destination file if it already exists.

cat myfile.txt
# This is my file

cp -n myfile.txt myfile2.txt

The -n option modifies the default behavior of the cp command.

cat myfile2.txt
# This is my 2nd file

Modifying output

Other options modify a commands output.

Example 1

ls -a lists all files, including hidden ones (those starting with a dot). This option alters the command’s output by showing files that are not listed by default.

ls -a data
# .
# ..
# README.md
# music_vids.tsv
# pwrds.tsv
# raw
# trees.tsv
# vg_hof.tsv
# wu_tang.psv
# wu_tang.tsv

Example 2

df -h shows disk space usage in human-readable form (e.g., KB, MB, GB), modifying the default output to be more easily understood.

df -h
# Filesystem                          Size    Used   Avail Capacity iused ifree %iused  Mounted on
# /dev/disk1s1s1                     466Gi   9.5Gi    40Gi    20%    404k  422M    0%   /
# devfs                              201Ki   201Ki     0Bi   100%     696     0  100%   /dev
# /dev/disk1s3                       466Gi   2.4Gi    40Gi     6%    5.1k  422M    0%   /System/Volumes/Preboot
# /dev/disk1s5                       466Gi   1.0Gi    40Gi     3%       1  422M    0%   /System/Volumes/VM
# /dev/disk1s6                       466Gi    19Mi    40Gi     1%      19  422M    0%   /System/Volumes/Update
# /dev/disk1s2                       466Gi   411Gi    40Gi    92%    6.3M  422M    1%   /System/Volumes/Data
# map auto_home                        0Bi     0Bi     0Bi   100%       0     0     -   /System/Volumes/Data/home

Environment-specific options

The -u option in sort removes duplicate lines. -us behavior (considering case sensitivity) might vary depending on the locale and environment settings.

Example

sort -u data/roxanne.txt sorts the lines in data/roxanne.txt, removing duplicate lines.¹

sort -u data/raw/roxanne.txt
# I have to tell you just how I feel
# I know my mind is made up
# I loved you since I knew you
# I won't share you with another boy
# I wouldn't talk down to you
# It's a bad way
# Ro...
# Roxanne
# Roxanne (Put on the red light)
# Roxanne (You don't have to put on the red light)
# So put away your make up
# Those days are over
# Told you once I won't tell you again
# Walk the streets for money
# You don't care if it's wrong or if it's right
# You don't have to put on the red light
# You don't have to sell your body to the night
# You don't have to wear that dress tonight

Combining Options

Multiple short options can be combined after a single dash, without spaces (e.g., -lrt) allowing users to use multiple options at once, and reducing the need to type multiple dashes.

Multiple short options

Options can be combined with arguments when they are followed by a space and then the argument (e.g., -o filename).

Example

ls -lrt combines three options: -l (long listing format), -r (reverse order), and -t (sort by modification time), providing a detailed, reverse-chronological list of files.

ls -lrt data
# total 112
# -rw-r--r--@  1 mjfrigaard  staff    263 Apr 10 09:39 wu_tang.tsv
# -rw-r--r--@  1 mjfrigaard  staff   4417 Apr 10 14:01 trees.tsv
# -rw-r--r--@  1 mjfrigaard  staff   6122 Apr 10 14:04 music_vids.tsv
# -rw-r--r--   1 mjfrigaard  staff   4814 Apr 10 14:07 vg_hof.tsv
# -rw-r--r--@  1 mjfrigaard  staff    462 Apr 15 14:07 wu_tang.psv
# -rw-r--r--@  1 mjfrigaard  staff  12057 Apr 22 14:11 pwrds.tsv
# drwxr-xr-x@ 10 mjfrigaard  staff    320 May 15 21:55 raw
# -rw-r--r--@  1 mjfrigaard  staff   8798 May 15 21:59 README.md

Options with values

Some options require or accept an argument to specify a value related to the option’s action.

Example

grep-i "FILE" myfile.txt uses -i to ignore case when searching for "FILE" in myfile.txt:

grep -i "FILE" myfile.txt
# This is my file

The "FILE" here is an argument for the -i option.

Recap

Options greatly enhance the power and versatility of Unix commands, allowing users to tailor operations to their specific needs and preferences.

Not all Unix-like systems or shells may support the same options for a given command, and behavior can vary between implementations. It’s important to refer to a command’s manual page (using man command or command --help) for the most accurate and comprehensive list of options and their effects.

See a typo, error, or something missing?

Please open an issue on GitHub.

rm myfile.txt
rm myfile2.txt

data/roxanne.txt contains the lyrics to the 1978 song Roxanne by The Police.↩︎