Core utilities

Related articles

• Bash
• Zsh
• General recommendations
• GNU Project
• sudo
• cron
• File system search
• man page
• Securely wipe disk#shred
• File permissions and attributes

This article deals with so-called core utilities on a GNU/Linux system, such as less, ls, and grep. The scope of this article includes, but is not limited to, those utilities included with the GNU coreutils package. What follows are various tips and tricks and other helpful information related to these utilities.

Basic commands

The following table lists basic shell commands every Linux user should be familiar with. Commands in bold are part of the shell, others are separate programs called from the shell. See the below sections and Related articles for details.

Command	Description	Example
man	Show manual page for a command	man ed
cd	Change directory	cd /etc/pacman.d
mkdir	Create a directory	mkdir ~/newfolder
rmdir	Remove empty directory	rmdir ~/emptyfolder
rm	Remove a file	rm ~/file.txt
rm -r	Remove directory and contents	rm -r ~/.cache
ls	List files	ls *.avi
ls -a	List hidden files	ls -a /home/archie
ls -al	List hidden files and file properties
mv	Move a file	mv ~/compressed.zip ~/archive/compressed2.zip
cp	Copy a file	cp ~/.bashrc ~/.bashrc.bak
chmod +x	Make a file executable	chmod +x ~/.local/bin/myscript.sh
cat	Show file contents	cat /etc/hostname
strings	Show printable characters in binary files	strings /usr/bin/free
find	Search for a file	find ~ -name myfile
mount	Mount a partition	mount /dev/sdc1 /media/usb
df -h	Show remaining space on all partitions
ps -A	Show all running processes
killall	Kill all running instances of a process

cat

cat (catenate) is a standard Unix utility that concatenates and lists files.

• Because cat is not a built-in shell, on many occasions you may find it more convenient to use a redirection, for example in scripts, or if you care a lot about performance. In fact < file does the same as cat file.

• cat is able to work with multiple lines, although this is sometimes regarded as bad practice:

$ cat << EOF >> path/file
first line
...
last line
EOF

A better alternative is the echo command:

$ echo "\
first line
...
last line" \
>> path/file

• If you need to list file lines in reverse order, there is a utility called tac (cat reversed).

dd

dd is a command on Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.

Note: cp does the same as dd without any operands but is not designed for more versatile disk wiping procedures.

Checking progress of dd while running

By default, there is no output of dd until the task has finished. With kill and the USR1 signal you can force status output without actually killing the program. Open up a second root terminal and issue the following command:

# killall -USR1 dd

Note: This will affect all other running dd processes as well.

Or:

# kill -USR1 pid_of_dd_command

For example:

# kill -USR1 $(pidof dd)

This causes dd to output immediate progress into the terminal. For example:

605+0 records in
605+0 records out
634388480 bytes (634 MB) copied, 8.17097 s, 77.6 MB/s

Using pipe viewer

As an alternative you can use pv to monitor the dd-pipeline:

# dd if=/source/filestream | pv -monitor_options -s size_of_file | dd of=/destination/filestream

To use the pipe viewer more easily you can add this to your bashrc or zshrc:

copy() {
    size=$(stat -c%s $1)
    dd if=$1 &> /dev/null | pv -petrb -s $size | dd of=$2
}

And use it with:

# copy /source/file /destination/file

dd spin-offs

Other dd-like programs feature periodical status output, e.g. a simple progress bar.

dcfldd 

dcfldd is an enhanced version of dd with features useful for forensics and security. It accepts most of dd's parameters and includes status output. The last stable version of dcfldd was released on December 19, 2006.^[1]

ddrescue 

GNU ddrescue is a data recovery tool. It is capable of ignoring read errors, which is a useless feature for disk wiping in almost any case. See the official manual for details.

grep

grep (from ed's g/re/p, global/regular expression/print) is a command line text search utility originally written for Unix. The grep command searches files or standard input globally for lines matching a given regular expression, and prints them to the program's standard output.

• Remember that grep handles files, so a construct like cat file | grep pattern is replaceable with grep pattern file

• grep alternatives optimized for VCS source code do exist, such as the_silver_searcher and ack.

Colored output

grep's color output can be helpful for learning regexp and additional grep functionality.

To enable grep coloring write the following entry to the shell configuration file (e.g. if using Bash):

~/.bashrc

alias grep='grep --color=auto'

To include file line numbers in the output, add the option -n to the line.

The environment variable GREP_COLOR can be used to define the default highlight color (the default is red). To change the color find the ANSI escape sequence for the color liked and add it:

export GREP_COLOR="1;32"

GREP_COLORS may be used to define specific searches.

Standard error

Some commands send their output to standard error, and grep has no apparent effect. In this case, redirect standard error next to standard out:

$ command 2>&1 | grep args

or Bash 4 shorthand:

$ command |& grep args

See also I/O Redirection.

iconv

iconv converts the encoding of characters from one codeset to another.

The following command will convert the file foo from ISO-8859-15 to UTF-8 saving it to foo.utf:

$ iconv -f ISO-8859-15 -t UTF-8 foo >foo.utf

See man iconv for more details.

Convert a file in place

Tip: You can use recode instead of iconv if you do not want to touch the mtime.

Unlike sed, iconv does not provide an option to convert a file in place. However, sponge can be used to handle it, it comes with moreutils.

$ iconv -f WINDOWS-1251 -t UTF-8 foobar.txt | sponge foobar.txt

See man sponge for details.

ip

ip allows you to show information about network devices, IP addresses, routing tables, and other objects in the Linux IP software stack. By appending various commands, you can also manipulate or configure most of these objects.

Note: The ip utility is provided by the iproute2 package, which is included in the base group.

Object	Purpose	Manual Page Name
ip addr	protocol address management	ip-address
ip addrlabel	protocol address label management	ip-addrlabel
ip l2tp	tunnel Ethernet over IP (L2TPv3)	ip-l2tp
ip link	network device configuration	ip-link
ip maddr	multicast addresses management	ip-maddress
ip monitor	watch for netlink messages	ip-monitor
ip mroute	multicast routing cache management	ip-mroute
ip mrule	rule in multicast routing policy db
ip neigh	neighbour/ARP tables management	ip-neighbour
ip netns	process network namespace management	ip-netns
ip ntable	neighbour table configuration	ip-ntable
ip route	routing table management	ip-route
ip rule	routing policy database management	ip-rule
ip tcp_metrics	management for TCP Metrics	ip-tcp_metrics
ip tunnel	tunnel configuration	ip-tunnel
ip tuntap	manage TUN/TAP devices
ip xfrm	manage IPsec policies	ip-xfrm

The help command is available for all objects. For example, typing ip addr help will show you the command syntax available for the address object. For advanced usage see the iproute2 documentation.

The Network configuration article shows how the ip command is used in practice for various common tasks.

Note: You might be familiar with the ifconfig command, which was used in older versions of Linux for interface configuration. It is now deprecated in Arch Linux; you should use ip instead.

less

less is a terminal pager program used to view the contents of a text file one screen at a time. Whilst similar to other pagers such as more and pg, less offers a more advanced interface and complete feature-set.

See List of applications#Terminal pagers for alternatives.

Colored output through environment variables

Add the following lines to your shell configuration file:

~/.bashrc

export LESS=-R
export LESS_TERMCAP_me=$(printf '\e[0m')
export LESS_TERMCAP_se=$(printf '\e[0m')
export LESS_TERMCAP_ue=$(printf '\e[0m')
export LESS_TERMCAP_mb=$(printf '\e[1;32m')
export LESS_TERMCAP_md=$(printf '\e[1;34m')
export LESS_TERMCAP_us=$(printf '\e[1;32m')
export LESS_TERMCAP_so=$(printf '\e[1;44;1m')

Change values as you like. References: ANSI escape code.

Colored output through wrappers

You can enable code syntax coloring in less. First, install source-highlight, then add these lines to your shell configuration file:

~/.bashrc

export LESSOPEN="| /usr/bin/source-highlight-esc.sh %s"
export LESS='-R '

Frequent users of the command line interface might want to install lesspipe.

Users may now list the compressed files inside of an archive using their pager:

$ less compressed_file.tar.gz

==> use tar_file:contained_file to view a file in the archive
-rw------- username/group  695 2008-01-04 19:24 compressed_file/content1
-rw------- username/group   43 2007-11-07 11:17 compressed_file/content2
compressed_file.tar.gz (END)

lesspipe also grants less the ability of interfacing with files other than archives, serving as an alternative for the specific command associated for that file-type (such as viewing HTML via python-html2text).

Re-login after installing lesspipe in order to activate it, or source /etc/profile.d/lesspipe.sh.

Vim as alternative pager

Vim (visual editor improved) has a script to view the content of text files, compressed files, binaries, directories. Add the following line to your shell configuration file to use it as a pager:

~/.bashrc

alias less='/usr/share/vim/vim74/macros/less.sh'

There is also an alternative to less.sh macro, which may work as the PAGER environment variable. Install vimpager and add the following to your shell configuration file:

~/.bashrc

export PAGER='vimpager'
alias less=$PAGER

Now programs that use the PAGER environment variable, like git, will use vim as pager.

Colored output when reading from stdin

Note: It is recommend to add #Colored output through environment variables to your ~/.bashrc or ~/.zshrc, as the below is based on export LESS=R

When you run a command and pipe its standard output (stdout) to less for a paged view (e.g. pacman -Qe | less), you may find that the output is no longer colored. This is usually because the program tries to detect if its stdout is an interactive terminal, in which case it prints colored text, and otherwise prints uncolored text. This is good behaviour when you want to redirect stdout to a file, e.g. pacman -Qe > pkglst-backup.txt, but less suited when you want to view output in less.

Some programs provide an option to disable the interactive tty detection:

# dmesg --color=always | less

In case that the program does not provide any similar option, it is possible to trick the program to believe that its stdout is an interactive terminal with the following utilities:

• stdoutisatty — A small program which catches the isatty function call.

https://github.com/lilydjwg/stdoutisatty. || stdoutisatty-git^AUR

Example: stdoutisatty program | less

• unbuffer — A script based on sh and Tcl.

http://expect.sourceforge.net/example/unbuffer.man.html || expect

Example: unbuffer program | less

Alternatively, using zpty module from zsh: [2]

~/.zshrc

zmodload zsh/zpty

pty() {
	zpty pty-${UID} ${1+$@}
	if [[ ! -t 1 ]];then
		setopt local_traps
		trap '' INT
	fi
	zpty -r pty-${UID}
	zpty -d pty-${UID}
}

ptyless() {
	pty $@ | less
}

Usage:

$ ptyless program

To pipe it to other pager (less in this example):

$ pty program | less

ls

ls (list) is a command to list files in Unix and Unix-like operating systems.

• ls can list file permissions.

• Colored output can be enabled with a simple alias. File ~/.bashrc should already have the following entry copied from /etc/skel/.bashrc:

alias ls='ls --color=auto'

The next step will further enhance the colored ls output; for example, broken (orphan) symlinks will start showing in a red hue. Add the following to your shell configuration file:

eval $(dircolors -b)

mkdir

mkdir (make directory) is a command to create directories.

• To create a directory and its whole hierarchy, the -p switch is used, otherwise an error is printed. As users are supposed to know what they want, -p switch may be used as a default:

alias mkdir='mkdir -p -v'

The -v switch make it verbose.

• Changing mode of a just created directory using chmod is not necessary as the -m option lets you define the access permissions.

Tip: If you just want a temporary directory, a better alternative may be mktemp (make termporary): mktemp -p.

mv

mv (move) is a command to move and rename files and directories.

• It can be very dangerous so it is prudent to limit its scope:

alias mv=' timeout 8 mv -iv'

This alias suspends mv after eight seconds, asks confirmation to delete three or more files, lists the operations in progress and does not store itself in the shell history file if the shell is configured to ignore space starting commands.

rm

rm (remove) is a command to delete files and directories.

• It can be very dangerous, so it is prudent to limit its scope:

alias rm=' timeout 3 rm -Iv --one-file-system'

This alias suspends rm after three seconds, asks confirmation to delete three or more files, lists the operations in progress, does not involve more than one file systems and does not store itself in the shell history file if the shell is configured to ignore space starting commands. Substitute -I with -i if you prefer to confirm even for one file.

Zsh users may want to put noglob before timeout to avoid implicit expansions.

• To remove directories known to be empty, use rmdir as it fails in case of files inside the target.

sed

sed (stream editor) is a Unix utility that parses and transforms text.

Here is a handy list of sed one-liners examples.

Tip: More powerful alternatives are AWK and even Perl language.

seq

seq (sequence) is a utility for generating a sequence of numbers. Shell built-in alternatives are available, so it is good practice to use them as explained on Wikipedia.

which

The which command is useful to determine the path to an executable, for example:

# journalctl $(which sshd)

motd

motd (Message of the day). The contents of /etc/motd are displayed by login(1) after a successful login but just before it executes the login shell.

It is a good place to display your Terms of Service to remind users of your local policies or anything you wish to tell them.

See also

• A sampling of coreutils , part 2 , part 3 - Overview of commands in coreutils

• GNU Coreutils Manpage

• Learn the DD command