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the directories, but it also means that it is always outdated.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
-
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
-
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
-
Write a find command that finds all files created after January 30th 2010.
-
Write a find command that finds all *.odf files created in September 2009.
-
Count the number of *.conf files in /etc and all its subdirs.
-
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/ \;
-
Create a file called
loctest.txt
. Can you find this file withlocate
? Why not ? How do you make locate find this file ? -
Use find and -exec to rename all .htm files to .html.
-
Issue the
date
command. Now display the date in YYYY/MM/DD format. -
Issue the
cal
command. Display a calendar of 1582 and 1752. Notice anything special ?
solution: basic Unix tools
-
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.When
*.txt
is quoted then the shell will not touch it. Thefind
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). Thefind
might show a different result, or can result in a syntax error. -
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?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.
-
Write a find command that finds all files created after January 30th 2010.
-
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 asnot newer
. -
Count the number of *.conf files in /etc and all its subdirs.
-
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.The first might fail when there are too many files to fit on one command line.
-
Create a file called
loctest.txt
. Can you find this file withlocate
? Why not ? How do you make locate find this file ?
You cannot locate this with locate
because it is not yet in the index.
```bash
updatedb
```
-
Use find and -exec to rename all .htm files to .html.
-
Issue the
date
command. Now display the date in YYYY/MM/DD format. -
Issue the
cal
command. Display a calendar of 1582 and 1752. Notice anything special?Remark that the calendars are different depending on the locale.