Difference between revisions of "Bash operators"
imported>ThorstenStaerk m (Reverted edits by 221.178.182.30 (talk) to last revision by 173.183.218.83) |
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now is a valid bash command which overrides one of these files with the other one. | now is a valid bash command which overrides one of these files with the other one. | ||
| − | + | = $() = | |
| + | The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily. | ||
| + | |||
| + | Examples | ||
| + | <source> | ||
| + | rpm -ql $(rpm -qa) | ||
| + | |||
| + | for i in $(seq 1 1 100); do echo $i; done | ||
| + | </source> | ||
= $(()) = | = $(()) = | ||
| Line 14: | Line 22: | ||
Examples: | Examples: | ||
| + | <source> | ||
# echo $((2*2)) | # echo $((2*2)) | ||
4 | 4 | ||
| + | </source> | ||
= $$ = | = $$ = | ||
| Line 21: | Line 31: | ||
Example: | Example: | ||
| + | <source> | ||
# kill -9 $$ | # kill -9 $$ | ||
| + | </source> | ||
kills the current process | kills the current process | ||
| − | + | = $! = | |
| + | The operator $! delivers the id of the process that has most recently sent to the [[background]]. | ||
| + | |||
| + | Example: | ||
| + | <source> | ||
| + | # [[xosview]] & [[sleep]] 5; [[kill]] $! | ||
| + | </source> | ||
| + | shows xosview for 5 seconds and stops it | ||
= > and >> = | = > and >> = | ||
| Line 31: | Line 50: | ||
Examples: | Examples: | ||
| + | <source> | ||
ls -alh > files-in-directory.txt | ls -alh > files-in-directory.txt | ||
ls -alh /otherdir >> files-in-directory.txt # Append list of files in other directory | ls -alh /otherdir >> files-in-directory.txt # Append list of files in other directory | ||
| + | </source> | ||
= 2> or 2>> = | = 2> or 2>> = | ||
| Line 41: | Line 62: | ||
Examples: | Examples: | ||
| + | <source> | ||
cat notexisting 2> test # Writes "File or directory not found." to test | cat notexisting 2> test # Writes "File or directory not found." to test | ||
cat notexisting 2>&1 # Outputs "File or directory not found." to stdout | cat notexisting 2>&1 # Outputs "File or directory not found." to stdout | ||
| + | </source> | ||
= | = | = | = | ||
| is used to pipe the stdout of one programm to the stdin of another one. | | is used to pipe the stdout of one programm to the stdin of another one. | ||
| + | <source> | ||
tail -n 100 ''foo'' | sort -r # Reverse sort the last 100 lines of the file ''foo'' | tail -n 100 ''foo'' | sort -r # Reverse sort the last 100 lines of the file ''foo'' | ||
| + | </source> | ||
= && = | = && = | ||
&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write | && evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write | ||
| + | <source> | ||
command1 && command2 | command1 && command2 | ||
| + | </source> | ||
command2 will only be executed if command1 returned success. | command2 will only be executed if command1 returned success. | ||
Revision as of 05:42, 3 April 2020
You have a shell script and wonder what these operators do? Then this article is for you.
Contents
* and ?
You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.
These wildcards are actually bash operators! Imagine, we have the files a1, a2:
cp a*
now is a valid bash command which overrides one of these files with the other one.
$()
The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily.
Examples <source>
rpm -ql $(rpm -qa)
for i in $(seq 1 1 100); do echo $i; done
</source>
$(())
The operator $(()) in the bash shell is replaced by the arithmetic result of the expression enclosed in the parentheses.
Examples: <source>
# echo $((2*2)) 4
</source>
$$
The operator $$ delivers the id of the currently-running process.
Example: <source>
# kill -9 $$
</source> kills the current process
$!
The operator $! delivers the id of the process that has most recently sent to the background.
Example: <source>
# xosview & sleep 5; kill $!
</source> shows xosview for 5 seconds and stops it
> and >>
> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.
Examples: <source>
ls -alh > files-in-directory.txt ls -alh /otherdir >> files-in-directory.txt # Append list of files in other directory
</source>
2> or 2>>
stderr can be redirected by using 2> or 2>>.
But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).
Examples: <source>
cat notexisting 2> test # Writes "File or directory not found." to test cat notexisting 2>&1 # Outputs "File or directory not found." to stdout
</source>
|
| is used to pipe the stdout of one programm to the stdin of another one. <source>
tail -n 100 foo | sort -r # Reverse sort the last 100 lines of the file foo
</source>
&&
&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write <source>
command1 && command2
</source> command2 will only be executed if command1 returned success.
