Skip to content

Latest commit

 

History

History
242 lines (230 loc) · 6.37 KB

text_commands.md

File metadata and controls

242 lines (230 loc) · 6.37 KB
Raw

Text commands

Linux provides utilities for file and text manipulation:

  1. Display contents using cat and echo.
  2. Edit file contents using sed and awk.
  3. Search for patterns using grep.

Display contents

The cat is short for concatenate and is often used to read and print files as well as for simply viewing file contents, while the tac command prints the lines of a file in reverse order.

$ cat > myfile.txt
Mario Rossi
Antonio Esposito
Michele Laforca
Ctrl-D
$ cat myfile.txt
Mario Rossi
Antonio Esposito
Michele Laforca
$
$ tac myfile.txt
Michele Laforca
Antonio Esposito
Mario Rossi

The echo simply displays text.

$ echo myfile.txt
myfile.txt
]$echo HOME
HOME
$ echo $HOME
/home/ec2-user

Edit file content

The command sed is a powerful text processing tool. Its name is an abbreviation for stream editor. It filters text as well as perform substitutions in data streams. Data from an input source/file (or stream) is taken and moved to a working space. The entire list of operations/modifications is applied over the data in the working space and the final contents are moved to the standard output space (or stream).

$ sed s/Mario/Saverio/ myfile.txt
Saverio Rossi
Antonio Esposito
Michele Laforca
$ cat myfile.txt
Mario Rossi
Antonio Esposito
Michele Laforca
$ sed s/Mario/Saverio/ myfile.txt >  myfile2.txt
$ cat myfile2.txt
Saverio Rossi
Antonio Esposito
Michele Laforca
$ sed -i s/Mario/Saverio/ myfile.txt
$ cat myfile.txt
Saverio Rossi
Antonio Esposito
Michele Laforca

For example, to convert 01/02/… to JAN/FEB/…

sed -e 's/01/JAN/' -e 's/02/FEB/' -e 's/03/MAR/' -e 's/04/APR/' -e 's/05/MAY/' \ 
-e 's/06/JUN/' -e 's/07/JUL/' -e 's/08/AUG/' -e 's/09/SEP/' -e 's/10/OCT/' \
-e 's/11/NOV/' -e 's/12/DEC/'

The awk command is used to extract and then print specific contents of a file and is often used to construct reports. It is a powerful utility and interpreted programming language, used to manipulate data files, retrieving, and processing text. It works well with fields (containing a single piece of data, essentially a column) and records (a collection of fields, essentially a line in a file).

$ awk '{ print $0 }' myfile.txt
Saverio Rossi
Antonio Esposito
Michele Laforca
$ awk '{ print $1 }' myfile.txt
Saverio
Antonio
Michele
$ awk '{ print $2 }' myfile.txt
Rossi
Esposito
Laforca

Please, check the man pages for the awk and sed commands for futher details.

File manipulation

The sort command is used to rearrange the lines of a text file either in ascending or descending order, according to a sort key.

# cat myfile.txt
Mario Rossi
Antonio Esposito
Michele Laforca
# sort myfile.txt
Antonio Esposito
Mario Rossi
Michele Laforca
# sort -r myfile.txt
Michele Laforca
Mario Rossi
Antonio Esposito

The uniq is used to remove duplicate lines in a text file and is useful for simplifying text display. It requires that the duplicate entries to be removed are consecutive.

# cat myfile.txt
Mario Rossi
Antonio Esposito
Michele Laforca
Antonio Esposito
# sort myfile.txt | uniq
Antonio Esposito
Mario Rossi
Michele Laforca
# sort myfile.txt | uniq -c
      2 Antonio Esposito
      1 Mario Rossi
      1 Michele Laforca

The paste command is used to combine fields from different files

# cat names.txt
Mario Rossi
Antonio Esposito
Michele Laforca
Antonio Esposito
[root@caldera01 ~]# cat ages.txt
34
46
29
46
[root@caldera01 ~]# paste names.txt ages.txt
Mario Rossi     34
Antonio Esposito        46
Michele Laforca 29
Antonio Esposito        46

The join command combines two files on a common field

# cat names.txt
01 Mario Rossi
02 Antonio Esposito
03 Michele Laforca
04 Antonio Esposito
# cat ages.txt
01 34
02 46
03 29
04 46
# join names.txt ages.txt
01 Mario Rossi 34
02 Antonio Esposito 46
03 Michele Laforca 29
04 Antonio Esposito 46

The grep comand is extensively used as a primary text searching tool. It scans files for specified patterns and can be used with regular expressions.

# grep Ant* names.txt
02 Antonio Esposito
04 Antonio Esposito

The tr utility is used to translate specified characters into other characters or to delete them.

# cat names.txt
01 Mario Rossi
02 Antonio Esposito
03 Michele Laforca
04 Antonio Esposito
# cat names.txt | tr a-z A-Z
01 MARIO ROSSI
02 ANTONIO ESPOSITO
03 MICHELE LAFORCA
04 ANTONIO ESPOSITO

The tee command takes the output from any command, and while sending it to standard output, it also saves it to a file.

# ls -l | tee list.txt
total 32
-rw-r--r--. 1 root root   24 Mar  3 14:42 ages.txt
-rw-------. 1 root root 1883 Jan 21 20:53 anaconda-ks.cfg
-rw-r--r--. 1 root root   74 Mar  3 14:42 names.txt
-rwxr--r--. 1 root root  102 Feb 21 16:47 script.sh
-rw-r--r--. 1 root root   74 Mar  3 14:52 tr
[root@caldera01 ~]# cat list.txt
total 32
-rw-r--r--. 1 root root   24 Mar  3 14:42 ages.txt
-rw-------. 1 root root 1883 Jan 21 20:53 anaconda-ks.cfg
-rw-r--r--. 1 root root   74 Mar  3 14:42 names.txt
-rwxr--r--. 1 root root  102 Feb 21 16:47 script.sh
-rw-r--r--. 1 root root   74 Mar  3 14:52 tr

The wc (word count) counts the number of lines, words, and characters in a file or list of files.

# cat names.txt
01 Mario Rossi
02 Antonio Esposito
03 Michele Laforca
04 Antonio Esposito
[root@caldera01 ~]# wc -l names.txt
4 names.txt
[root@caldera01 ~]# wc -c names.txt
74 names.txt
[root@caldera01 ~]# wc -w names.txt
12 names.txt

The cut command is used for manipulating column-based files and is designed to extract specific columns. The default column separator is the tab character. A different delimiter can be given as a command option.

# cut -d" " -f1 names.txt
01
02
03
04
# cut -d" " -f2 names.txt
Mario
Antonio
Michele
Antonio
# cut -d" " -f3 names.txt
Rossi
Esposito
Laforca
Esposito

The head reads the first few lines of each named file (10 by default) and displays it on standard output.

# head -n 2 names.txt
01 Mario Rossi
02 Antonio Esposito

The tail prints the last few lines of each named file and displays it on standard output. By default, it displays the last 10 lines.

# tail -n 2 names.txt
03 Michele Laforca
04 Antonio Esposito
#
# tail -f -n3 /var/log/messages
Mar  3 14:38:59 caldera01 systemd: Started Session 35 of user root.
Mar  3 15:01:01 caldera01 systemd: Starting Session 36 of user root.
Mar  3 15:01:01 caldera01 systemd: Started Session 36 of user root.