196 lines
3.9 KiB
Markdown
196 lines
3.9 KiB
Markdown
# STIN, STOUT, STERR
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Input is 0, output is 1, error is 2.
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Pipe standard output to log.txt while also outputting it.
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> cat file.txt |& tee -a log.txt
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Copy file and *if* that's successful, delete it where it stands.
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> scp archive.tar.gz pi@192.168.0.31:/home/pi && rm archive.tar.gz
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A double pipe will try one, and do the other if that fails.
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> cp -r ~/Archive ~/Backup || tar czf Archive.tar.gz *
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# REGEX
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Regular expression characters include:
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\\ ^ $ . | ? * + () [] {}
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As a result, grep cannot read these characters as literal characters unless they are escaped. E.g.
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> grep wtf\\? log.txt
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... will search the string 'wtf?' in the file log.txt. Another version is egrep (now used with 'grep -e') which uses more characters as special characters, or fgrep, which treats all characters as literal strings.
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# Environmental Variables
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PWD, USER, PATH
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To display all environmental (but not local) variables, use
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> env
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Set a variable with
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> colour=red
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Display your variable with
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> echo $colour
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Export this to the entire system using:
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> export colour=blue
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# Search for Programs
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Search for commands relevant to `cat`.
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> apropos cat
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Show files used in the `cat` program.
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> whereis cat
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Show *which* file is the actual code which runs when you type `cat`:
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> which cat
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# Working with Text
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Convert every tab to ten spaces.
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> expand -t 10 file.txt
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Or the reverse, with 3 spaces converting to a tab.
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> unexpand -t 3 file.txt
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Format a file by cutting text after 60 characters.
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> fmt -w 60 file.txt
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Indent all but the first line of a paragraph.
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> fmt -t file.txt
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Look at the new lines of a file only:
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> tail -f /var/log/syslog
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The sort function arranges lines alphabetically. Use -r to reverse and -n to sort by number.
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# Sed
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Change all examples of hey to hoi in greetings and show that output (does not change file).
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> sed 's/hey/hoi/g greetings.txt'
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Change each example of 'cat' to 'dog' in the file 'animals.md'.
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> sed 's/cat/dog/g' animals.md
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# Measurement
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Measure how long a script takes for super-autism powers.
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> time [bash script]
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# Aliases
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Make one word equal a longer command with `alias`.
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E.g.:
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> alias wot='sudo systemd-analyze blame && free -h '
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Now when you type 'go', it will perform that entire operation.
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# Functions
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Make a function which checks if something is a file, and if so, shows it on screen with `cat`:
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> function show(){
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> file "$1" | grep text &\>/dev/null && cat "$1"
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> }
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That `$1` refers to the first argument typed after the command.
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If you want to run this on a file called `list`, then use:
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> show list
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...and the list will output, only if it is text.
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In total, this functions the same as typing:
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> file list | grep text &\>/dev/null && list
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Remove a function with:
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> unset my_function
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# Paths
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Every shell has various paths from where it can execute binary files. Find out your current one with:
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> echo $PATH
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To add a directory to a path, e.g. /usr/share/bin, you can declare it in addition to the old path with:
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> PATH=$PATH:/usr/share/bin
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And then check it by echoing the path again.
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Before this, probably best to check the path exists with:
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> if [ -e /usr/share/bin ]; then
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> echo yes
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> fi
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# Pipes, Pedantry and Brackets
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Things that [[ ]] statements can do which [ ] statements cannot:
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- Intuitive and easy 'and' statements.
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- [[ -z $var && -d ~/LK ]]
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- Intuitive and easy 'or' statements.
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- [[ -d LK || -f ghost-backup.zip ]]
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- Simple expression comparisons
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- [[ $v1 > $v2 ]]
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- Simple expression comparisons with clumsy strings
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- [[ item-1 > item-2 ]]
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- Vague comparisons
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- [[ $answer =~ ^y(es)?$ ]]
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# exec
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exec will start a process running as just that process. In a bash script, the line:
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> unison rat
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... will startup `unison` as a sub-process of bash. But:
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> exec unison rat
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... starts unison as its own process.
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# Brace expansion
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> mv picture{,-1}.jpg
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This expands to
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> mv picture.jpg picture-1.jpg
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# `for` Statements
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for f in *tiff;do
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convert "$f" "${f/.tiff/.png}"
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done
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