Core utilities

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zh-cn:Core utilities zh-tw:Core utilities

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 *.mkv
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
ss -at Display a list of open TCP sockets

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.

By default, dd outputs nothing until the task has finished. To monitor the progress of the operation, add the status=progress option to the command. See the manual for more information.

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

dd spin-offs

Tango-go-next.pngThis article or section is a candidate for moving to Disk cloning.Tango-go-next.png

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

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.

find

find is part of the findutils package, which belongs to the base package group.

One would probably expect a find command to take as argument a file name and search the filesystem for files matching that name. For a program that does exactly that see #locate below.

Instead, find takes a set of directories and matches each file under them against a set of expressions. This design allows for some very powerful "one-liners" that would not be possible using the "intuitive" design described above. See UsingFind for usage details.

locate

locate is a common Unix tool for quickly finding files by name. It offers speed improvements over the find tool by searching a pre-constructed database file, rather than the filesystem directly. The downside of this approach is that changes made since the construction of the database file cannot be detected by locate. This problem is minimised by regular, typically scheduled use of the updatedb command, which (as the name suggests) updates the database.

Note: Although in other distros locate and updatedb are in the findutils package, they are no longer present in Arch's package. To use it, install the mlocate package. mlocate is a newer implementation of the tool, but is used in exactly the same way.

Before locate can be used, the database will need to be created. To do this, simply run updatedb as root.

See also How locate works and rewrite it in one minute

Keeping the database up-to-date

When mlocate is installed, a script is automatically scheduled to run daily via systemd, to update the database. You can also manually run updatedb as root at any time.

To save time, the updatedb can be (and by default is) configured to ignore certain filesystems and paths by editing /etc/updatedb.conf. man updatedb.conf will tell you about the semantics of this file. It is worth noting that among the paths ignored in the default configuration (i.e. those in the "PRUNEPATHS" string) are /media and /mnt, so locate may not discover files on external devices.

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

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements. See Help:Style for reference.Tango-edit-clear.png

Reason: less is a complex beast, and this section should explain some of the basic less commands - not go on a bunch of tangents like colored output (Discuss in Talk:Core utilities#)

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_mb=$'\E[1;31m'
export LESS_TERMCAP_md=$'\E[1;36m'
export LESS_TERMCAP_me=$'\E[0m'
export LESS_TERMCAP_se=$'\E[0m'
export LESS_TERMCAP_so=$'\E[01;44;33m'
export LESS_TERMCAP_ue=$'\E[0m'
export LESS_TERMCAP_us=$'\E[1;32m'

Change the values (ANSI escape code) as you like.

Note: The LESS_TERMCAL_xx variables is currently undocumented in less(1), for a detailed explanation on these sequences, see this anwser.

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 recommended 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 into thinking 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-gitAUR
Example: stdoutisatty program | less
  • unbuffer — A tclsh script comes with expect, it invokes desired program within a pty.
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.

See info ls or the online manual for more information.

Long format

The -l option displays some metadata, for example:

$ ls -l /path/to/directory
total 128
drwxr-xr-x 2 archie users  4096 Jul  5 21:03 Desktop
drwxr-xr-x 6 archie users  4096 Jul  5 17:37 Documents
drwxr-xr-x 2 archie users  4096 Jul  5 13:45 Downloads
-rw-rw-r-- 1 archie users  5120 Jun 27 08:28 customers.ods
-rw-r--r-- 1 archie users  3339 Jun 27 08:28 todo
-rwxr-xr-x 1 archie users  2048 Jul  6 12:56 myscript.sh

The total value represents the total disk allocation for the files in the directory, by default in number of blocks.

Below, each file and subdirectory is represented by a line divided into 7 metadata fields, in the following order:

  • type and permissions:
    • the first character is the entry type, see info ls -n "What information is listed" for an explanation of all the possible types; for example:
      • - denotes a normal file;
      • d denotes a directory, i.e. a folder containing other files or folders;
      • p denotes a named pipe (aka FIFO);
      • l denotes a symbolic link;
    • the remaining characters are the entry's permissions;
  • number of hard links for the entity; files will have at least 1, i.e. the showed reference itself; folders will have at least 2: the showed reference, the self-referencing . entry, and then a .. entry in each of its subfolders;
  • owner user name;
  • group name;
  • size;
  • last modification timestamp;
  • entity name.

Colored output

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)

File names containing spaces enclosed in quotes

By default, file and directory names that contain spaces are displayed surrounded by single quotes. To change this behavior use the -N or --quoting-style=literal options. Alternatively, set the QUOTING_STYLE environment variable to literal. [3]

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 temporary): 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.

od

The od (octal dump) command is useful for visualizing data that is not in a human-readable format, like the executable code of a program, or the contents of an unformatted device. See the manual for more information.

pv

You can use pv (pipe viewer) to monitor the progress of data through a pipeline, for example:

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

In most cases pv functions as a drop-in replacement for cat, however there are undocumented differences. For example, under both Zsh and Bash, the following command hangs forever:

# cat <(pv /usr/share/dict/words)

Use of strace shows that pv is stopped with SIGTTOU.

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)

wipefs

wipefs can list or erase file system, RAID or partition-table signatures (magic strings) from the specified device. It does not erase the file systems themselves nor any other data from the device.

See wipefs(8) for more information.

For example, to erase all signatures from the device /dev/sdb and create a signature backup ~/wipefs-sdb-offset.bak file for each signature:

# wipefs --all --backup /dev/sdb

See also