Extras: More Useful Linux Commands

While we cover the basic file and navigation commands in Lab 1, There are many more commands that are available as standard on a Linux system that can make your life much easier. In this file we briefly go through some of the more commonly used ones and list a few useful options or potential use cases for each. We’ll revisit many of these during the course as they’ll help us automate some aspects of running and extracting data from our calculations.

Some additional useful file commands

As you are all on the same system and file space is not unlimited, it is good to be aware of how much space you’re using. Large calculations can often take more disk space than you might have expected, so it’s important to be able to track down large files that are no longer needed if space becomes an issue. The first few commands here can help you with this, beyond using ls -lh to see how big files are.

df

df is used to report file system disk space usage for all mounted partitions. This is a useful way to check how much space is free on the disks and partitions used to store files by the system. It also tells you which disks are are real local disks: these are the ones with labels like “/dev/sda2” under the filesystem heading, while disks mounted across the network will be labelled with the server address in this field. The “tmpfs” label indicates a temporary filesystem, usually this is stored in RAM while the system is running, but can be used through a directory in the file hierarchy.

You might notice a remote server directory is mounted to ~/homedir. This your windows home directory. You can copy files and folders there if you would like to make the available on your Imperial windows account.

Useful Options

• df -h will list usage numbers in a human-readable format, i.e. instead of 10240, it would say “10M”.
• df -T will list the filesystem type e.g. ext3/ext4 for disks using a typical Linux filestructure, and nfs or cifs for disks mounted remotely over the network. It is useful to be able to identify filesystems that are mounted over the network since these are usually slower to use than local disks if your calculation produces many files. In these cases, if your home directory is on a remote filesystem you should run your calculation using a local disk and copy the results to your home directory once it is completed.

du

du is used to estimate file space usage. Typing du filename will tell you how big filename is (usually in kB by default). du dirname will tell you how big all the files in directory dirname and its subdirectories are, with a total at the end.

Useful Options

• du -s will only print the total size used for the argument. This is typically used to find how much space a given directory is using.
• du -h will print sizes in human-readable format.

quota

quota is used to check disk usage and limits. Many shared Linux systems, such as HPC systems, and the server you are using for this course, will impose a limit on the disk space used by any individual users. The quota command will tell you what those limits are and how much you have used.

Useful Options

• quota -s will show limits and usage in a human-readable format, i.e. instead of 3145180 it would say 3072M.

head and tail

head outputs the first few lines of a file to the terminal, while tail outputs the last few lines of a file to the terminal.

• head filename outputs the first 10 lines of filename.
• head -5 filename outputs the first 5 lines of filename.
• tail filename outputs the last 10 lines of filename.
• tail -5 filename outputs the last 5 lines of filename.
• tail -f filename starts tail in “follow” mode, where tail will repeatedly check for new data written to the file, and output them to the terminal. This is useful for following the output from a running calculation for example.

chmod

Permissions

If you take a look at the output of ls -l you’ll see the first column has a mix of letters (usually d, r, w, x, and hyphens).

• The first character in this column indicates the file: d for directories, and - for regular files.
• The following nine characters indicate the permissions. These are in sets of three where each set indicates the permission for a set of users.
  • The first set of three characters are the permissions for the user that owns the file (listed in the third column).
  • The second set of three characters are the permissions of other members of the group that own the file (listed in the fourth column).
    • The default on many Linux systems is to create a group of the same name as the username, that contains only that user.
    • It’s also possible for users to be added to several groups by the system administrator. This is useful on shared systems where a certain set of users want to share access to some set of files, but without giving access to everyone.
    • You can see what groups you are in by typing groups in the terminal.
  • The third set of three characters are the permissions for all other users.
• Within each set of three characters:
  • The first character is “r” if users in that set have permission to read the file and “-” otherwise.
  • The second character is “w” if users in that set have permission to write to the file and “-” otherwise.
  • The third character is “x” if users in that set have permission to exectute to the file, i.e. run it as a program.

chmod command

chmod can be used to change file permissions. This command can be invoked in two different ways:

One can change the permission for a given set of users granularly: - chmod u+x filename grants the user who owns the file execute permission. This is one of the main things you will be using chmod for as when you create a script in a text editor it will not be executable by default. - chmod g+rw filename grants group members read and write permission. - chmod o-r filename revokes other users read permission. - chmod a+x filename grants all users execute permission.

One can use a set of three numbers to set the full list of permissions at once. What each number corresponds to is listed in following table:

#	Permission	rwx
7	read, write and execute	rwx
6	read and write	rw-
5	read and execute	r-x
4	read only	r–
3	write and execute	-wx
2	write only	-w-
1	execute only	–x
0	none	—

• To set the permissions of a directory so only the owner can access it in any way you could use chmod 700 directoryname.
• To set a script you have created so that others can use and execute it you could use chmod 755 scriptname.

wc

wc filename will output the newline, word and byte counts for filename.

Useful Options

• -l to output the number of lines in the file.
• -w to output the word count for the file.

grep

grep will print lines from a file or stdin which match a given pattern.

• grep searchtext filename will output all lines in filename which contain the text searchtext.
• history | grep less will output all lines in the command history containing less. This is useful for those times when you entered a complex command some time ago that you want to repeat.

While searchtext in the first example above could be a particular word you want to find an exact match of, grep will also interpret this as a regular expression be default. This is somewhat similar to the wildcards you can use in the terminal, but has a slightly different syntax and allows for much more complex patterns.

Regular Expressions

This is a very deep topic, so we’ll only cover a few of the more simple examples. man grep has significantly more detail. The most useful symbols are probably:

• . matches any single character.
• * the preceding item will be matched zero or more times.
• These are quite useful when combined to form .*, which acts in the same way as the terminal wildcard expression *.
• Note: to match the actual . or * symbols, you can escape them as \. and \*.

For example grep "doc.*\.pdf" dirfiles.dat will output all lines containing strings that begin with doc and end with .pdf.

Note regular expressions can also be used in less (and hence man) when searching for text with /.

Useful Options

• grep -3 searchtext filename will additionally output 3 lines before and after any lines containing searchtext. Any number of lines can be used here.
• grep -v searchtext filename will output all lines except those containing searchtext.
• grep -r searchtext will recursively search all files and folders starting from the current directory.

cut

cut prints selected parts from each line a file. Mostly this is used with the -f option which tells it to print only certain fields, and the -d option which allows you to set the delimiter for fields (TAB is the default). It is often useful to pipe (|) the output of grep into cut to parse data from an output file.

For example, cut -d ' ' -f 1 filename will print the first word (separated by spaces) on each line of filename, and cut -d ',' -f '3-5' data.csv would print the 3rd, 4th and 5th columns of a csv data file.

Useful Options

• -s tells cut not to output lines which do not contain delimiters. This is useful for example if you have empty lines in the file you are parsing that you want to suppress.
• --complement will output the complement of the selected fields. For example, if we had a 5 column csv data file, cut -d ',' -f '2-4' --complement would output the 1st and 5th columns.

awk

awk is a pattern scanning and processing language. It is typically used to parse files in a similar manner to using grep combined with cut. awk is very powerful but we will only cover some very basic operations.

• awk '/regexp/{print}' filename will output all lines in filename containing regexp. As with grep, regular expressions can be used in regexp.
• awk '/regexp/{print $1" "$3}' filename will output the first and third words in all lines containing regexp. Note by default awk uses spaces as the field delimiter.
• awk 'BEGIN{i=0} /regexp/{i=i+1} END{print i}' filename will output the number of lines in filename containing regexp.
• awk '/searchtext/{printf "%f %f\n",$2-13.0,$4*10.0}' filename will output for each line containing searchtext, the second field with 13.0 subtracted, and the fourth field times 10.

Hopefully these examples give you an idea of what is possible with awk. More details and examples can be found with man awk.

Useful Options

• -F allows you to set the field separator. For example awk -F',' would be useful for parsing a csv file.
• -f program-file tells awk to run the commands listed in program-file. This is useful if you have a complicated script so you don’t need to type it all in directly to the terminal.

sed

sed stands for stream editor. It allows you to perform basic text transformations on a file (or from stdin). sed is very powerful and we will only cover some very simple examples.

• sed 's/regexp/replacement/' filename > newfile will replace the first match of regexp on each line of filename with replacement. Here the first s stands for substitute, and is probably the most useful sed command. Note that sed outputs to stdout by default, so you should redirect this to a new file to save. Do not try to redirect output to the same file you are reading, as > will blank the file before sed can read anything from it.
  • sed 's/^...//' filename > newfile will remove the first three characters from every line of filename. Note ^ is used to match the beginning of a line.
• sed 's/regexp/replacement/g' filename > newfile will replace every match of regexp on each line of filename with replacement.
  • sed 's/,/\t/g' data.csv > data.dat would replace all commas with tabs (\t is a tab) in data.csv and save it in data.dat.
• The -i flag can be used to modify a file in-place. sed -i 's/regexp/replacement/g' filename will replace every match of regexp on each line of filename with replacement. filename itself will be modified. You can also specify a suffix here such that a backup will be created using that suffix: e.g. sed -i.bak 's/regexp/replacement/g' filename will do the replacement in-place but first backup the original file to filename.bak.

See man sed for more information.

tr

tr is used to translate or delete characters. It always reads from stdin, and outputs to stdout. This means that to use it with a file, we need to redirect the file to stdin using <.

• tr 1 2 < test.dat would output the contents of test.dat with all the 1s replaced by 2s.
• tr abc ABC < test.txt would output the contents of test.txt with any ‘a’ replaced by ‘A’, ‘b’ by ‘B’ and ‘c’ by ‘C’.

It also accepts some special input such as

• [:space:] to match whitespace (both single and continuous).
• [:punct:] to match punctuation
• [:lower:] to match lower case letters
• [:upper:] to match upper case letters

For example:

• tr [:lower:] [:upper:] < test.txt > test_upper.txt would to create a new version test.txt converted to uppercase.
• tr [:space:] '\n' < test.txt would convert all spaces to newlines.

Useful options

• -d deletes matching characters. For example, to output a file with all punctuation removed we could do tr -d [:punct:] < test.txt

find

find is used to search for files in a directory hierarchy. Most commonly this is used with the -name option to search for files with a particular name. Wildcards can be used in the search. Note: the first argument to find should be the path to search, e.g. find /etc to search for files in the /etc directory or find . to search for files in the current directory.

• find . -name "*.cpp" will find all files ending in .cpp in the current directory (.) and its subdirectories.

See man find for more information.

diff

diff is used to compare two files. This is useful if for example, you want to see what changes have been made in a new version of a file.

• diff file1 file2 will output the lines which differ between the two files. The lines from file1 will be prepended with < and the lines from file2 with >.

Useful Options

• -q will report only whether the two files differ and will not output the differences.
• -r will recursively compare files in subdirectories.
• -y will output the two files side by side in two columns.
• -W will allow you to set how wide the output is (130 columns by default). This is particularly useful with the -y option.

sort

sort is used to sort lines of text files. For example, if we had a file called users.txt which contained a list of names, then sort users.txt would output (to stdout) the list sorted alphabetically. This is often useful combined with other commands. For example to generate a sorted list of all words in a file you can do sed 's/ /\n/g' filename | sort. Here the sed command replaces all spaces with new lines, so we have one word per line, and then we use this as input to the sort command.

Useful Options

• -n will sort numerically rather than alphabetically. For example, du -s * | sort -n will generate a listing of files and directories sorted by size.
• -h will use a human numeric sort, allowing numbers such as 2K and 1G to be sorted. For example, du -sh * | sort -h will generate a listing of files and directories sorted by size, but in human readable format.
• -u will output only the first of an equal run. For example sed 's/ /\n/g' filename | sort -u will generate a sorted list of all words in filename with each listed only once.
• -f will till sort to fold lower case to upper case characters.
• -r will output in reverse order. This is useful for numeric sorts where you often want to have the largest numbers at the top.

uniq

uniq is used to report or omit repeated lines in a file. By default it will take any file or input from stdin, and output it with duplicated lines omitted. For example if we had a text file test.txt with

a
a
b
b
b
c
d
d

Running uniq test.txt would output

a
b
c
d

Useful Options

• -c prefixes each line of output with a count of its number of occurrences. We could for example, take the sort example to sort all words in a file, and expand it to generate a word count of all words in a file: sed 's/ /\n/g' filename | sort | uniq -c. We could add | sort -n to the end of this to sort words in order of frequency, and we could use tr before this to remove all punctuation.
• -i tells uniq to ignore differences in case when comparing.

tar

tar is an archiving utility used to create an archive of files, i.e. generate a file containing many other files. This is usually used to create compressed bundles of files on Linux, in a similar way to zip file archives (note zip and unzip are usually available on Linux also, but compressed tar archives are more commonly used).

Creating Archives

The -c flag indicates to tar that you want to create an new archive.

• tar -cvf archive.tar file1 file2 dir1 will create an (uncompressed) archive called archive.tar of the named set of files or directories. Here:
  • -v is for verbose mode - it will list the files which are added. This is not really necessary, but is useful so you can be sure you are adding the files you intended to.
  • -f is used to specify the archive name. Here we have called it archive.tar.
• tar -czvf archive.tar.gz file1 file2 dir1 uses the additional -z flag to compress the archive using gzip compression. The extension .tar.gz or .tgz is typically used to indicate this type of file. Several other possible compression algorithms could be used instead:
  • -j will use bzip2 compression. This typically results in slightly smaller file size than gzip, but can take slightly longer. Files created like this usually use the extension .tar.bz or .tbz or some other similar variation.
  • -J will use xz compression. This is a very effective compression algorithm, resulting in very small files, particularly for archives containing a lot of text. This option can take quite a lot longer than gzip for large archives however. Files created with this option usually use the extension .tar.xz or .txz.

Extracting Archives

The -x flag indicates to tar you want to unpack an archive.

• tar -xvf archivename will uncompress and unpack a tar archive called archivename, automatically detecting what kind of compression was used (if any). Again the -v isn’t necessary, but is useful.

Listing Archive Content

The -t flag will tell tar to list the contents of an archive.

• tar -tvf archivename will list the contents of the tar archive archivename, again automatically detecting what kind of compression was used.

bc

bc is an arbitrary precision calculator language. This can be used from the command line by piping expressions to it. For example:

• echo "2+2" | bc
• For floating point operations, you should set the scale which defines how many digits following the decimal points are output:
  • echo "scale=10; 1.412*27.211" | bc
  • echo "scale=10; sqrt(2)" | bc
• You can tell bc to load the standard math library with the -l flag. This will also set the scale to 20 by default. This makes several additional functions available such as s(x), c(x) and a(x) for the sine, cosine and arctan in radians. So echo "4*a(1)" | bc -l will output the first 20 decimal places of pi.