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 GNUpackage. What follows are various tips and tricks and other helpful information related to these utilities.
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.
|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 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
< filedoes the same as
- 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 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.
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 patternis replaceable with
grep pattern file
- grep alternatives optimized for VCS source code do exist, such as and .
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 is part of thepackage, which belongs to the 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.
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. By default, paths such as
/mnt are ignored, so
locate may not discover files on external devices. See
man updatedb.conf for details.
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.
locate can be used, the database will need to be created. To do this, simply run
updatedb as root.
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
$ iconv -f ISO-8859-15 -t UTF-8 foo >foo.utf
man iconv for more details.
Convert a file in place
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 .
$ iconv -f WINDOWS-1251 -t UTF-8 foobar.txt | sponge foobar.txt
man sponge for 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.
|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|
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.
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.
ls is a command to list files in Unix and Unix-like operating systems.
info ls or the online manual for more information.
-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
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;
ddenotes a directory, i.e. a folder containing other files or folders;
pdenotes a named pipe (aka FIFO);
ldenotes a symbolic link;
- the remaining characters are the entry's permissions;
- the first character is the entry type, see
- 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;
- last modification timestamp;
- entity name.
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
--quoting-style=literal options. Alternatively, set the
QUOTING_STYLE environment variable to
mkdir makes 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
-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.
mv moves and renames files and directories.
To limit potential damage caused by the command, use an alias:
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.
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.
You can use(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
rm removes files or directories.
To limit potential damage caused by the command, use an alias:
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 if you prefer to confirm even for one file.
Zsh users may want to put
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.
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.
As an early Unix compression format,
tar files (known as tarballs) are widely used for packaging in Unix-like operating systems. Both pacman and AUR packages are tarballs, and Arch uses GNU's
Tar program by default.
tar by default will extract the file according to its extension:
$ tar xvf file.EXTENSION
Forcing a given format:
|File Type||Extraction Command|
The construction of some of these
tar arguments may be considered legacy, but they are still useful when performing specific operations. The Compatibility section of
tar's man page shows how they work in detail.
The which command is useful to determine the path to an executable, for example:
# journalctl $(which sshd)
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