basic Unix tools

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

This chapter introduces commands to find or locate files and to compress files, together with other common tools that were not discussed before. While the tools discussed here are technically not considered filters, they can be used in pipes.

find

The find command can be very useful at the start of a pipe to search for files. Here are some examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your screen with error messages.

Find all files in /etc and put the list in etcfiles.txt

find /etc > etcfiles.txt

Find all files of the entire system and put the list in allfiles.txt

find / > allfiles.txt

Find files that end in .conf in the current directory (and all subdirs).

find . -name "*.conf"

Find files of type file (not directory, pipe or etc.) that end in .conf.

find . -type f -name "*.conf"

Find files of type directory that end in .bak .

find /data -type d -name "*.bak"

Find files that are newer than file42.txt

find . -newer file42.txt

Find can also execute another command on every file found. This example will look for *.odf files and copy them to /backup/.

find /data -name "*.odf" -exec cp {} /backup/ \;

Find can also execute, after your confirmation, another command on every file found. This example will remove *.odf files if you approve of it for every file found.

find /data -name "*.odf" -ok rm {} \;

locate

The locate tool is very different from find in that it uses an index to locate files. This is a lot faster than traversing all the directories, but it also means that it is always outdated. If the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux) with the updatedb command.

[student@linux ~]$ locate Samba
warning: locate: could not open database: /var/lib/slocate/slocate.db:...
warning: You need to run the 'updatedb' command (as root) to create th...
Please have a look at /etc/updatedb.conf to enable the daily cron job.
[student@linux ~]$ updatedb 
fatal error: updatedb: You are not authorized to create a default sloc...
[student@linux ~]$ su -
Password: 
[root@linux ~]# updatedb
[root@linux ~]#

Most Linux distributions will schedule the updatedb to run once every day.

date

The date command can display the date, time, time zone and more.

student@linux ~$ date
Sat Apr 17 12:44:30 CEST 2010

A date string can be customised to display the format of your choice. Check the man page for more options.

student@linux ~$ date +'%A %d-%m-%Y'
Saturday 17-04-2010

Time on any Unix is calculated in number of seconds since 1969 (the first second being the first second of the first of January 1970). Use date +%s to display Unix time in seconds.

student@linux ~$ date +%s
1271501080

When will this seconds counter reach two thousand million ?

student@linux ~$ date -d '1970-01-01 + 2000000000 seconds'
Wed May 18 04:33:20 CEST 2033

cal

The cal command displays the current month, with the current day highlighted.

student@linux ~$ cal
     April 2010     
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

You can select any month in the past or the future.

student@linux ~$ cal 2 1970
   February 1970    
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28

sleep

The sleep command is sometimes used in scripts to wait a number of seconds. This example shows a five second sleep.

student@linux ~$ sleep 5
student@linux ~$

time

The time command can display how long it takes to execute a command. The date command takes only a little time.

student@linux ~$ time date
Sat Apr 17 13:08:27 CEST 2010

real    0m0.014s
user    0m0.008s
sys     0m0.006s

The sleep 5 command takes five real seconds to execute, but consumes little cpu time.

student@linux ~$ time sleep 5

real    0m5.018s
user    0m0.005s
sys     0m0.011s

This bzip2 command compresses a file and uses a lot of cpu time.

student@linux ~$ time bzip2 text.txt

real    0m2.368s
user    0m0.847s
sys     0m0.539s

gzip - gunzip

Users never have enough disk space, so compression comes in handy. The gzip command can make files take up less space.

student@linux ~$ ls -lh text.txt 
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
student@linux ~$ gzip text.txt 
student@linux ~$ ls -lh text.txt.gz 
-rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz

You can get the original back with gunzip.

student@linux ~$ gunzip text.txt.gz 
student@linux ~$ ls -lh text.txt
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt

zcat - zmore

Text files that are compressed with gzip can be viewed with zcat and zmore.

student@linux ~$ head -4 text.txt 
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
student@linux ~$ gzip text.txt 
student@linux ~$ zcat text.txt.gz | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh

bzip2 - bunzip2

Files can also be compressed with bzip2 which takes a little more time than gzip, but compresses better.

student@linux ~$ bzip2 text.txt 
student@linux ~$ ls -lh text.txt.bz2 
-rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2

Files can be uncompressed again with bunzip2.

student@linux ~$ bunzip2 text.txt.bz2 
student@linux ~$ ls -lh text.txt 
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt

bzcat - bzmore

And in the same way bzcat and bzmore can display files compressed with bzip2.

student@linux ~$ bzip2 text.txt 
student@linux ~$ bzcat text.txt.bz2 | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh

practice: basic Unix tools

1. Explain the difference between these two commands. This question is very important. If you don\'t know the answer, then look back at the shell chapter.

find /data -name "*.txt"

find /data -name *.txt

2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ?

find /data -name "*.odf" > data_odf.txt

find /data/*.odf > data_odf.txt

3. Write a find command that finds all files created after January 30th 2010.

4. Write a find command that finds all *.odf files created in September 2009.

5. Count the number of *.conf files in /etc and all its subdirs.

6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question.

cp -r /data/*.odf /backup/

find /data -name "*.odf" -exec cp {} /backup/ \;

7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ?

8. Use find and -exec to rename all .htm files to .html.

9. Issue the date command. Now display the date in YYYY/MM/DD format.

10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?

solution: basic Unix tools

1. Explain the difference between these two commands. This question is very important. If you don\'t know the answer, then look back at the shell chapter.

find /data -name "*.txt"

find /data -name *.txt

When *.txt is quoted then the shell will not touch it. The find tool will look in the /data for all files ending in .txt.

When *.txt is not quoted then the shell might expand this (when one or more files that ends in .txt exist in the current directory). The find might show a different result, or can result in a syntax error.

2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ?

find /data -name "*.odf" > data_odf.txt

find /data/*.odf > data_odf.txt

The first find will output all .odf filenames in /data and all subdirectories. The shell will redirect this to a file.

The second find will output all files named .odf in /data and will also output all files that exist in directories named *.odf (in /data).

With two million files the command line would be expanded beyond the maximum that the shell can accept. The last part of the command line would be lost.

3. Write a find command that finds all files created after January 30th 2010.

touch -t 201001302359 marker_date
find . -type f -newer marker_date

There is another solution :
find . -type f -newerat "20100130 23:59:59"

4. Write a find command that finds all *.odf files created in September 2009.

touch -t 200908312359 marker_start
touch -t 200910010000 marker_end
find . -type f -name "*.odf" -newer marker_start ! -newer marker_end

The exclamation mark ! -newer can be read as not newer.

5. Count the number of *.conf files in /etc and all its subdirs.

find /etc -type f -name '*.conf' | wc -l

6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question.

cp -r /data/*.odf /backup/

find /data -name "*.odf" -exec cp {} /backup/ \;

The first might fail when there are too many files to fit on one command line.

7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ?

You cannot locate this with locate because it is not yet in the index.

updatedb

8. Use find and -exec to rename all .htm files to .html.

student@linux ~$ find . -name '*.htm'
./one.htm
./two.htm
student@linux ~$ find . -name '*.htm' -exec mv {} {}l \;
student@linux ~$ find . -name '*.htm*'
./one.html
./two.html

9. Issue the date command. Now display the date in YYYY/MM/DD format.

date +%Y/%m/%d

10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?

cal 1582

The calendars are different depending on the country. Check http://linux-training.be/files/studentfiles/dates.txt