Bash Index : R - Httqm's Docs

print the resolved path

realpath someFile
```
/absolute/path/to/someFile
```

Synchronize yum repositories to a local directory

Flag	Default value	Usage
-a arch --arch=arch	current	download packages for the arch architecture
-d --delete		delete local packages no longer present in repository
--download-metadata		download all the non-default metadata
-p destDir --download_path=destDir	current dir	download packages to destDir
-g --gpgcheck		Remove packages that fail GPG signature checking after downloading exit status is `1` if at least one package was removed
-n --newest-only		download only newest packages per-repo
-l --plugins		enable yum plugin support
-r repoId --repoid=repoId	all	query the repository identified as repoId can be specified multiple times
-u --urls		list urls of what would be downloaded, don't download can serve as a dry-run

reposync --arch=x86_64 --newest-only --download-metadata --gpgcheck --plugins --repoid=rhel-7-server-rpms --repoid=rhel-7-server-extras-rpms --repoid=rhel-7-server-optional-rpms --download_path=/data

The Red Hat Package Manager

The man differentiates :

a package name : myPackage
from a package file : myPackage-<version>-<arch>.rpm

Flag	Usage
-e packageName --erase packageName	erase (i.e. uninstall) packageName
-i packageName --install packageName	install packageName with --nodeps : skip dependencies check
-U packageName --upgrade packageName	ugrade packageName = install new version + remove old version
-q [selectOptions] [queryOptions] --query [selectOptions] [queryOptions]	query the RPM database, with : selectOptions : -a --all : query all installed packages -f someFile --file someFile : display package owning file someFile -p packageName --package packageName : query an (uninstalled) package --whatrequires : list packages that require the specified package (source, opposite of rpm -qR) queryOptions : -l --list : list files of installed package see also : rpm -qi --filesbypkg packageName (source) -R --requires : list packages required by the specified package (opposite of rpm -q --whatrequires)

list files provided by a package: rpm -q l p packageFile

When the terminal is f*cked up because :

you ran : cat someBinaryFile

you may recover it with :

reset

It's possible that what you type is not echo'ed to the screen, but it'll work anyway .

return n

Causes a function to stop executing and return n —as a status code— to its caller. If n is omitted, the return status is that of the last command executed in the function body.
If return is used outside a function, but during execution of a script by the . (i.e. source) command, it causes the shell to stop executing that script and return either n or the exit status of the last command executed within the script as the exit status of the script. If n is supplied, the return value is its least significant 8 bits.
The return status is non-zero (i.e. "failure") if return is supplied a non-numeric argument, or is used outside a function and not during execution of a script by . or source.

Since return outputs status codes (aka exit codes), it allows calling a function and processing its return value with && and ||.

#!/usr/bin/env bash

returnParameter() { return $1; }

trueThenReturnParameter() { true; return $1; }

falseThenReturnParameter() { false; return $1; }

returnTrue() { true; return; }

returnFalse() { false; return; }

for testValue in 0 1 2 42 '' x; do
	returnParameter $testValue;          returnedValue1=$?
	trueThenReturnParameter $testValue;  returnedValue2=$?
	falseThenReturnParameter $testValue; returnedValue3=$?
	echo -e "test value : '$testValue'\treturned values : '$returnedValue1'\t'$returnedValue2'\t'$returnedValue3'\n"
done

echo
returnTrue && echo 'returned TRUE' || echo 'returned FALSE'
returnFalse && echo 'returned TRUE' || echo 'returned FALSE'

test value : '0'	returned values : '0'	'0'	'0'

test value : '1'	returned values : '1'	'1'	'1'

test value : '2'	returned values : '2'	'2'	'2'

test value : '42'	returned values : '42'	'42'	'42'

test value : ''		returned values : '0'	'0'	'1'

./test.sh: line 4: return: x: numeric argument required
./test.sh: line 9: return: x: numeric argument required
./test.sh: line 14: return: x: numeric argument required
test value : 'x'	returned values : '2'	'2'	'2'

returned TRUE
returned FALSE

Read

one line from the standard input or from a file, and assign the read value to a variable
keypresses (single key or combination of keys) and display the corresponding code (source) :
```
read
F2
^[OQ
Shift-F2
^[[1;2Q
Ctrl-F2
^[[1;5Q
```

You can do the same by running cat then typing keys.

Flag	Usage
-a myArray	"explode" substrings of the parameter into elements of myArray. Consider IFS to identify such substrings
-d delim	the 1^st character of delim is used to terminate the input line, rather than `newline`
-p prompt variableName	An example is worth 1000 words : read -p "What's your name ? " yourName; echo "Hello $yourName" What's your name ? `Bob` Hello Bob
-r	raw input : disables interpretation of backslash escapes and line-continuation in the read data

Read columns (source) :

This can be done by specifying several variables when calling read :

string='foo bar baz'; read a b c <<< "$string"; echo "$a $b $c"
text='one two three'; read -r col1 col2 col3 <<< "$text"; printf "col1: %s, col2: %s, col3: %s\n" "$col1" "$col2" "$col3"

Read data fields from a formatted text file :

tmpFile=$(mktemp tmp.XXXXXXXX); for i in {1..5}; do echo "${i}_1 ${i}_2 ${i}_3" >> "$tmpFile"; done; cat "$tmpFile"; while read itemA itemB itemC; do echo -e "$itemA\t$itemB\t$itemC"; done < "$tmpFile"; rm "$tmpFile"

Pop a group of lines from a file :

This will transpose this :

a
b
c

z

into this :

a	b	c
d	e	f
g	
y	z

tmpFile=$(mktemp tmp.XXXXXXXX); echo {a..z} | tr ' ' '\n' > "$tmpFile"; while true; do read a || break; read b; read c; echo -e "$a\t$b\t$c"; done < "$tmpFile"; rm "$tmpFile"

Display the previous and the current runlevels. If no previous runlevel is known, it will just display N.
On older systems (?), this information may be available in the RUNLEVEL and PREVLEVEL environment variables.

resize2fs [options] device size
Resize ext2, ext3, or ext4 file systems. It can be used to enlarge or shrink an unmounted file system.
If size parameter is not specified, it will default to the size of the partition.

Flag	Usage
-f	force resize2fs to proceed with the filesystem resize operation, overriding some safety checks it normally enforces

Shrink a partition :

resize2fs /dev/mapper/myLVM-homes 1800G

renice newNiceValue -p PID

Alter niceness (priority) of running processes. Instead of renice-ing a new process, it is wiser to nice it at startup.

Non-root users :

can only renice their own processes
can only increase the nice value (i.e. decrease priority) of a process (see example)

Flag	Usage
-n newNiceValue	set the new nice value to newNiceValue A niceness of −20 is the highest priority and 19 is the lowest priority. The default niceness a of process is inherited from its parent process, usually 0. The -n is optional, but must be the 1^st argument when used.
-p PID

Read the nice value of a process :

ps -o nice PID
top -p PID

Playing with renice :

sleep 3 & renice 5 -p $!

[1] 19145
19145 (process ID) old priority 0, new priority 5

sleep 3 & renice -n 5 -p $!

[1] 19167
19167 (process ID) old priority 0, new priority 5

Same as above since -n is optional.

sleep 3 & renice 5 -p $!; renice 5 -p $!

[1] 19208
19208 (process ID) old priority 0, new priority 5
19208 (process ID) old priority 5, new priority 5

You can not renice an already reniced process (or at least, not like this).

sleep 3 & renice +5 -p $!; renice +5 -p $!

[1] 19251
19251 (process ID) old priority 0, new priority 5
19251 (process ID) old priority 5, new priority 5

The + sign seems to make no big difference.

sleep 3 & renice 5 -p $!; renice 1 -p $!

[1] 19272
19272 (process ID) old priority 0, new priority 5
renice: failed to set priority for 19272 (process ID): Permission denied

The value we specify is NOT an increment, it's the expected value (see this example).
Only root can decrease a nice value (i.e. give higher priority to a process).

sleep 3 & renice 5 -p $!; renice 6 -p $!

[1] 20538
20538 (process ID) old priority 0, new priority 5
20538 (process ID) old priority 5, new priority 6

readlink mySymLink: return the target of the symbolic link mySymLink
readlink -f myFile: return the absolute path of myFile (source, see realpath)

Flag	Usage
-i	ask for confirmation before deleting each target file
-f	force delete
-r	delete recursively

/bin/rm: Argument list too long (source) :

cd directory; find . -name '*' -type f | xargs rm
cd directory; find . -name '*' -type f -delete

Rename files : rename from to files will rename the specified files by replacing the first occurrence of from in their name by to.

from and to can be specified as a substitution RegExp.
/usr/bin/rename is installed with the rename package.

Flag	Usage
-n	no action : show how files would have been renamed

The from / to method :

Given the files foo1, ..., foo9, foo10, ..., foo234, the commands :

rename foo foo0 foo?
rename foo foo0 foo??

will turn them into foo001, ..., foo009, foo010, ..., foo234.

Replace all spaces by underscores in filenames :

rename 's/ /_/g' *

Replace some substrings while keeping others, using callbacks :

The regular expressions have a callback syntax to refer to a "previously matched substring". In other tools (sed, Emacs, Vi, ...), such callbacks are written \n (n being the number of the matched expression, starting at 1). Even though rename accepts this syntax, it will complain saying it prefers them specified using the $ sign : $n. Example :

touch /tmp/file_01.txt; rename 's/(^.*_)..(\.txt)/${1}02$2/' /tmp/file_01.txt; ls -l /tmp/file_*

Here, curly braces are used to refer to the 1^st matched expression because the following characters are digits, and we don't want this reference to be interpreted as the 102^nd matched expression.
rename prefers the $n syntax, but we mustn't forget that a $ sign within double quotes will be expanded by the shell interpreter (Bash or others).

Nice numbering of ordered files :

Let's consider the set of files produced by :

prefix='testFile_'; extension='.ext'; for i in {1..20}; do touch "$prefix$i$extension"; done

For the files 1 to 9 to be numbered 01 to 09, just run :

prefix='testFile_'; rename 's/_([0-9]\.)/_0$1/g' $prefix*

$prefix is used to avoid affecting all files with a rename regExp *
the substring to match for re-use must be enclosed by ( ) (no need to escape parens), and is recalled as $n (not \n), n being the n^th matched substring

This hack _may_ turn useless with : {01..20}

Rename and number a collection of files :

Let's consider a collection of files :

file names are not consistent
extensions are not consistent either : mp3/MP3, jpg/jpeg/JPG, ...
files have no numbering
the relative numbers of files are not important

... in which we would like to decrease entropy. To do so, let's :

rename files based on a pattern
number files

Let's do it with :

prefix='myFile - '; extension='.jpg'; fileNb=1; for i in *; do rename "s/^.*$/$prefix$fileNb$extension/g" "$i"; fileNb=$((fileNb+1)); done; rename 's/ ([0-9]'$extension')/ 0$1/g' *

The rename after the for loop is to re-arrange file numbers nicely.

realpath

Usage

Example

reposync

Usage

Flags

Example

rpm

Usage

Flags

Example

reset

return

Usage

Example

read

Usage

Flags

Example

Read columns (source) :

Read data fields from a formatted text file :

Pop a group of lines from a file :

runlevel

resize2fs

Usage

Flags

Example

Shrink a partition :

renice

Usage

Flags

Example

Read the nice value of a process :

Playing with renice :

readlink

Usage

rm

Flags

Example

/bin/rm: Argument list too long (source) :

rename

Usage

Flags

Example

The from / to method :

Replace all spaces by underscores in filenames :

Replace some substrings while keeping others, using callbacks :

Nice numbering of ordered files :

Rename and number a collection of files :