Core utilities

Related articles

• Bash
• Zsh
• General recommendations
• GNU Project

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

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

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

cron

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.

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

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

Colored output

Beyond aesthetics, grep's color output is immensely useful for learning regexp and grep's functionality.

To use the default colors for grep, write the following entry to your shell configuration file, e.g. if using Bash:

~/.bashrc

alias grep='grep --color=auto'

Alternatively, you can set the GREP_OPTIONS environment variable bearing in mind this may break some scripts that use grep [1]:

export GREP_OPTIONS='--color=auto'

To include file line numbers in the output, add -n:

alias grep='grep -n --color=auto'

The environment variable GREP_COLORS may be used to specify different colors than the defaults.

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.

ip

Note that the ip command is in iproute2 package now.

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.

Object	Purpose	manpage
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.

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 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:

~/.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 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-git^AUR 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.

locate

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

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)

man

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

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 want just 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.

shred

shred is a command to securely delete files and directories. It 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

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

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