Linux shell commands
The linux shell command line interface is a powerful tool for performing system administration tasks. In comparison a graphical user interface (GUI) is easier to use but has limitations: (i) confined to perform specific tasks, (ii) required to navigate through menus and (iii) not easily configurable to perform batch jobs.
Some of the advantages in using the command line interface are:
- It is often faster to type a command on the keyboard than having to navigate menus in a GUI
- The command line interface usually requires less resources than a GUI
- Commands can be used in scripting to produce highly customisable and powerful tools to perform specific tasks
Many books and online resources are available on shell commands. As a starting point a self-contained manual page is available at the console by using the man command:
| man name_of_command |
For example the following command will provide details about the du command
|
$ man du DU(1) User Commands DU(1)
NAME
du - estimate file space usage
SYNOPSIS
du [OPTION]... [FILE]...
du [OPTION]... --files0-from=F
DESCRIPTION
Summarize disk usage of each FILE, recursively for directories.
Mandatory arguments to long options are mandatory for short options
too.
-a, --all
write counts for all files, not just directories
--apparent-size
print apparent sizes, rather than disk usage; although the
apparent size is usually smaller, it may be larger due to holes
in (`sparse') files, internal fragmentation, indirect blocks,
and the like
-B, --block-size=SIZE
use SIZE-byte blocks
etc... |
Command Line Reference
A summary of commonly used (bash shell) commands is tabulated below. Specific examples are given in the sections that follow.
| Basic Commands (Navigation and Disk Space) | |||
| cd | Change directory | ||
| cp | Copy files or directories | ||
| df | Report file system disk space usage | ||
| du | Estimate the amount of disk space used by file(s)/directories | ||
| ls | List directory contents | ||
| pwd | Display the name of the current directory | ||
| rm | Delete files or directories | ||
| Files and Permissions | |||
| cat | Concatenate files (print contents to standard output) | ||
| chmod | Set file permissions | ||
| chown | Set file ownership and group | ||
| diff | Compare the contents of two files and report the difference | ||
| file | Determine the file type | ||
| head | Display the first part of a file | ||
| less | View a file (with backward screen movement) | ||
| more | View a file (a screen at a time) | ||
| sort | Sort lines in a text file | ||
| stat | Display file details and attributes | ||
| tail | Display the last part of a file | ||
| Searching & String Manipulation | |||
| awk | Pattern scanning and processing | ||
| find | Search for files in a directory structure | ||
| grep | Search for patterns in a file | ||
| sed | Stream editor for performing text transformations | ||
| locate | Find files by name | ||
| System Administration & Diagnostics | |||
| dmesg | Print the contents of the kernel ring buffer (useful for troubleshooting hardware issues) | ||
| fdisk | Disk partition table tool | ||
| finger | Display user information | ||
| lsof | List open files | ||
| lspci | List PCI devices | ||
| ps | Reporting tool of current processes | ||
| kill | Send a signal to a running process (a terminate signal by default) | ||
| shutdown | Shutdown the system | ||
| top | Display linux tasks | ||
| uname | Print system information (e.g. hostname, kernel version) | ||
| mount | Mount a filesystem | ||
| w | Display currently logged in users | ||
| Archiving & Backups | |||
| rsync | File copy & synchronisation tool. Often used for backing up files to remote server | ||
| tar | Archiving utility | ||
| Networking & Remote Access | |||
| hostname | Display the system hostname | ||
| ifconfig | Configure a network interface | ||
| nslookup | Query a host | ||
| ping | Test the connectivity to a host | ||
| ssh | Secure shell access - Remote login program | ||
Examples
Basic Commands - Directories & Files
| ls | List the contents of the current directory |
| ls -lat | List of the contents of the current directory. Include hidden files, long listing format and sort by modification time |
| ls -R | List the contents of the current directory. Include all subdirectories in the long listing format |
| pwd | Display the name of the current directory |
| cd .. | Move up one directory level (from the current working directory) |
| cd /home/joe_blogs | Change the current working directory to /home/joe_blogs |
| cp imagetest.jpg imagetest2.jpg | Copy the file imagetest.jpg to imagetest2.jpg |
| cp -rp directory1 directory2 | Copy the directory1 (and all contents within) to directory2, preserving file permissions |
| mv movie1.iso movie2.iso | Rename the file movie1.iso to movie2.iso |
| rm module.h | Delete the file module.h |
| rm -r sqlite3 | Delete the directory sqlite3. The recurse option (-r) will delete the directory and all files contained within |
| mkdir Images | Create a directory called Images (in the current working directory) |
| rmdir fontconfig | Remove the directory fontconfig. This command will only work if the directory is empty |
Basic Commands - Disk Space Monitoring
| df | Show free space on all mounted file systems |
| df -h | Show free space on all mounted file systems in human readable format (e.g. MB, GB, ...) |
| df -h /home | Show the amount of free space on the file system /home in human readable format |
| du | Report the disk usage of all files in the current directory (recurse into sub directories) in kB |
| du -sh | Report the total disk usage of all files in the current directory (recurse into sub directories) in kB |
| du * | sort -nr | head -10 | Create a report of disk usage of all files in the current directory. Show the top 10 files (sorted by file size) |
Files & Permissions
| cat graphics.h | Print the contents of the file graphics.h to the screen |
| more /etc/profile | Display the contents of the file /etc/profile one screen full at a time |
| less textfile.dat | Display the contents of the file /etc/profile. Similar to more with the added feature that backward movement is allowed |
| head license.dat | Display the first part of the file license.dat |
| tail /etc/passwd | Display the last part of the file /etc/passwd. The default is to display the last 10 lines if no options are given |
| tail -f /var/log/messages | Display the last part of the file /var/log/messages with the follow option. The output will be updated as data is appended to the file |
| chmod g+rw archive.md5 | Set the group permission of the file archive.md5 to read and write |
| chmod 644 cerfiticate.pdf | Set the group permission of the file archive.md5 to read and write |
| chown -R john backups | Set the ownership of the directory backups to john. The recursive option (-R) has been selected to operate on all files and sub-directories inside the directory called backups |
| file patch.py | Report the file type (for the file patch.py). Some possible results include (i) ASCII text files, (ii) binaries and (iii) script executables |
| stat grid.bmp | Display the status of a file, i.e. whether it is a file or directory, permissions, modification time etc ... |
| sort -nr filename | Sort the contents of the filename using a numerical sort in reverse order |
| sort -k2 filename | Sort the contents of the filename by the 2nd field (the default field delimeter is a space) |
| diff file1 file2 | Report the differences between file1 and file2. Lines from file1 (not in file2) are displayed with < (at the beginning of each line). Lines in file2 (not in file1) are displayed with > |
| diff -y file1 file2 | Report the differences between file1 and file2. Display the output in 2 columns side by side |
Searching - Files
| find . -type f -name '*.pdf' | Search for files ending in .pdf in the current directory (and recurse into sub-directories) |
| find ~ -type f -name '*.doc' | wc -l | Count the number of files ending in .doc in the home directory (~). Note how the output of the find command is piped into the wc command |
| find /var/log -type f -mtime +365 | Search for files in /var/log that were modified over a year ago |
| locate *.jpg | Files all files ending in jpg (*.jpg) on the system. locate relies on a system database (updatedb) to be update daily by cron. |
Searching - Text file and string manipulation
| grep "joe" filename.txt | Search for the string "joe" in the file filename.txt |
| grep -A 3 fred /etc/passwd | Search for "fred" in the file /etc/passwd. If there is a match also display the next 3 lines |
| grep -i error report.log | Search for the string "error" in the file report.log. The -i option indicates that the match is case-insensitive, e.g. ERROR will be reported as a match |
| grep -v "apples" list.dat | Display lines in the file list.dat which don't match the pattern "apples". The -v option inverts the match |
| grep -v '^$' file.dat | Display the contents of the file file.dat with the exception of blank lines. The special characters ^ and $ refer to the beginning and the end of a line respectively |
| awk '{print $1}' file.dat | Print the 1st column of the contents of the file file.dat |
| ls -l | awk '{print $NF}' | Print the last field on the output of the command ls -l |
| awk -F":" '{print $3}' /etc/passwd | Print a list of all the user IDs on the current system, i.e. for each line in /etc/passwd print out the 3rd field. The field delimeter is the colon |
| sed 's/John/Bill/g' address.dat | Change all instances of the string "John" to "Bill" in the file address.dat. The command option "s" denotes substitution and the "g" denotes global replacement, i.e. make changes to the whole file |
| sed '/^$/d' x.dat | Remove all blank lines using the file x.dat as input data. ^ and $ are special characters denoting the beginning and end of a line respectively |
Archiving & Backups
| tar -cvf bin.tar bin | Create the tar archive file bin.tar of all files in the bin directory: Options: -c = create, -v = verbose output and -f specifies the tar file |
| tar -tf bin.tar | List the contents of the tar file bin.tar. Options: -t = list the contents, -f specifies the filename |
| tar -xvf bin.tar | Extract the contents of bin.tar into to the current directory. Options: -x = extract, -v = verbose output and -f specifies the tar file |
| rsync -av /directory /dest/path | Copy files from /directory to /dest/path (archive mode and verbose output) |
| rsync -avx /var /some/directory | Copy files from /var to /some/directory (archive mode, verbose output and don't cross file system boundaries) |
| rsync -av --delete /home/joe user@host:/backup | Copy files using ssh from /home/joe (archive mode & verbose output) to the directory /backup on the remote server with hostname host. Use user to authenticate and delete files at the destination that don't exist on the source machine (i.e. /home/joe) |
System Administration & Diagnostics
| top | Display linux tasks |
| ps -aef | Display all running processes in a full format listing |
| uptime | Report how long the system has been up and running (in days) |
| uname -a | Print system information such as hostname, kernel version, platform and processor type |
| dmesg | Print the kernel ring buffer. Useful for diagnosing hardware problems |
| cat /etc/issue | Report the operating system |
| passwd | Change the user password |
| id | Print the user ID of the current user |
| id user | Print the user (and group) ID of user |
| lsof | List all open files |
| lsof /data | List all open files in the filesystem /data |
| lsof -p 12345 | List all open files |
| lsof -u user123 | List all files that are currently open by the user user123 |
| kill 12345 | Send the terminal signal to process with ID 12345. This will attempt to kill the process (with signal id 15) |
| kill -9 12345 | Send the kill signal to the process with ID 12345. This is more aggressive than the previous example |
| exit | exit the current shell |
| shutdown -r now | Reboot the system immediately |
| finger username | Display information about the user username |
| su | Switch user (become super user) |
| su username | Switch to user username in the current login session |
| su - username | Switch to user username in the current login session. Setup the user environment as if the user username logged in directly |
| sudo command | Execute the command as another user. In this case there is no argument provided for the username (i.e. - username). The command will be executed as the root user |
| mount /home | mount the filesystem labelled /home (in /etc/fstab) |
| mount -t iso9660 /dev/sr0 /mnt/cd | mount a CD (with device name /dev/sr0) to the mount point /mnt/cd |
| w | Show a list of users currently logged into the system |
Networking & Remote Access
| hostname | Display the hostname (of the current session) |
| ifconfig -a | Display details of all network interfaces (even if they are down) |
| ifconfig eth1 192.168.2.1 | Assign 192.168.2.1 as the IP address for the network interface eth0 |
| ifconfig eth0 up | Enable the network interface eth0 |
| nslookup www.example.com | Query the name server www.example.com |
| nslookup 192.168.2.10 | Query the server with IP address 192.168.2.10 |
| ping 74.125.237.50 | ping the machine with IP address 74.125.237.50 |
| ping domainname.com | ping the mahine with DNS name domainname.com |
| ssh -l joe machinename | Connect using a secure shell login program to the host called machinename as the user joe |
| ssh -X -i key2 fred@machine | Connect using a secure shell login program to the host called machine with X-forwarding enabled as the user fred and using the private key key2 |
References and Related links
| A basic linux command guide | http://www.my-guides.net/en/content/view/29/26/ |
| A more comprehensive guide | http://www.pixelbeat.org/cmdline.html |
| Unix (Linux) tutorial for beginners | http://www.ee.surrey.ac.uk/Teaching/Unix/ |
Further Reading
- Linux Command Line and Shell Scripting Bible (Richard Blum)
- Learning the bash Shell (Cameron Newman)
- Unix and Linux System Administration Handbook (Evi Nemeth, Garth Snyder, Trent R. Hein and Ben Whaley)
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