Core utilities

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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.


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
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).


cron is a time-based job scheduler in Unix-like computer operating systems.

See the main article.

Note: systemd is able to handle many cron use cases. See the related article.


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.


# 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=$(ls -l $1 | awk '{print $5}')
    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 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]
GNU ddrescue is a data recovery tool. It's capable of ignoring read errors, which is a useless feature for disk wiping in almost any case. See the official manual for details.


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

Colored output

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

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

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.


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.


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

Colored output through environment variables

Add the following lines to your shell configuration file:

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:

export LESSOPEN="|| /usr/bin/ %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 html2text).

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

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:

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

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

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.

  • stdoutisatty — A small program which invokes a "fake interactive tty". || stdoutisattyAUR
Example: $ stdoutisatty stdout
  • socat — A relay for bidirectional data transfer between two independent data channels. It is based on GNU readline. || socat
Example: socat EXEC:"stdout",pty STDIO
  • script — An utility that makes typescript of a terminal session. || util-linux
Example: script -fqc "stdout" or [2]
  • unbuffer — A script based on sh and Tcl. || expect
Example: unbuffer stdout

Alternatively, using zpty from zsh: [3]

pty() {
	zmodload zsh/zpty
	zpty pty-${UID} ${1+"$@"}
	zpty -r pty-${UID}
	zpty -d pty-${UID}

ptyless() {
	pty $@ | less


$ ptyless stdout

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

$ pty stdout | less


locate serves to find files on filesystems. It searches through a prebuilt database of files generated by updatedb or by a daemon and compressed using incremental encoding. It operates significantly faster than find, but requires regular updating of the database.

See the main article.


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

  • 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)


man (manual page) is a form of online software documentation usually found on a Unix or Unix-like operating system. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts. See Man Pages.


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

  • To create a directory and its whole hierarchy, -p switch is used, if not a 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 (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 (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 (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 (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.


shred is a Unix command that can be used to securely delete files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all. shred uses three passes, writing pseudo-random data to the device during each pass. This can be reduced or increased.

The following command invokes shred with its default settings and displays the progress.

# shred -v /dev/sdX

Alternatively, shred can be instructed to do only one pass, with entropy from e.g. /dev/urandom.

# shred --verbose --random-source=/dev/urandom -n1 /dev/sdX

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

alias shred=' timeout 3 shred -v'

This alias suspends shred after three seconds, 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.

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


Sudo (as superuser do) is a program for Unix-like computer operating systems that allows users to run programs with the security privileges of another user (normally the superuser, or root). See Sudo.

Permissions related utilities

  • chmod (change mode) is the name of a Unix shell command and a system call, which both change the access permissions to file system objects (including files and directories), as well as specifying special flags.
  • chown (change owner) is used on Unix-like systems to change the owner of a file.
  • chattr (change attributes) is a command in the Linux operating system that allows a user to set certain attributes on a file residing on many Linux filesystems.
  • lsattr (list attributes) is a command-line program for listing the attributes on a Linux extended file system.
  • ls -l lists files attributes.

These utilities are explained in the File permissions and attributes article. More advanced permission use cases are satisfied by capabilities and ACL.

See also