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A shell is a UNIX system command processor. Basically, it is a command language to tell the system what to do. There are two major shell (command interpreter) families:
Bourne, Korn, Bash Shell
C Shell
The syntax of shell commands can vary from one shell to another.
At UQAM we use the Bourne Again Shell (bash). Therefore all commands below are bash commands.
Syntax of a shell command
command-name [ -option(s) filename(s) or arguments ]
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ls -l -t -r
is the same as
ls -lrt
In case you have a file name or an argument starting with a '-' you can use '--'. The command will NOT interpret anything following a '--' as an option/key.
For example if you are using 'grep' to look for '-a' in a file you could type:
$ grep -iw -- -a filename
For more detailed explanations about Program Argument Syntax Conventions, please click on the here.
Documentation of specific commands
whatis command - displays a one-line summary about command
man command - displays on-line manual pages about command
'man' can be use on almost all of the shell commands listed below to get the complete description of the command.
Shell commands are case sensitive
Type commands exactly as shown; most shell commands are lower case. File and directory names can be lower, upper, or mixed case but must be typed exactly as listed.
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| <Esc> . | repeat last word of previous command |
| Ctrl-a | jump to beginning of line |
| Ctrl-e | jump to end of line |
| Ctrl-r | search backwards in command history |
| Ctrl-c | interrupt current command/process |
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Conditional structures
if-then-else
| Volet |
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| if expression_1; then Statement(s) to be executed if expression 1 is true [elif expression_2; then Statement(s) to be executed if expression 2 is true elif expression_3; then Statement(s) to be executed if expression 3 is true else Statement(s) to be executed if no expression is true] fi |
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| Volet |
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Loops
Loops can be used to execute (parts of) code repeatedly.
for-loop
Loop over different fixed elements
| Volet |
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| for var in word1 word2 ... wordN; do Statement(s) to be executed for every word done |
Examples:
Loop over numbers:
| Volet |
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| for hour in 00 06 12 18 ; do echo $hour done |
Loop over files:
| Volet |
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| touch dm_01 dm_02 for fine in dm* ; do echo $file done |
while-loop
Repeat statement(s) while 'condition' is true.
| Volet |
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| while |
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| condition ; do Statement(s) to be executed while expression is true done |
Example:
| Volet |
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| a=1 while (( a <= 10 )) ; do echo $a a=$(( a + 1 )) # increase the loop parameter done |
You One can also use a while-loop to read lines from an ASCII file.
Example:
| Volet |
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| cat > text_file << EOF > Hello > How are you? |
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| EOF |
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| while read line ; do echo $line; done < text_file Hello How are you? |
break
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& continue
To interrupt a loop you can use 'break' and 'continue'.
break interrupts the loop
continue interrupts the current cycle of the loop
Manipulating Variables
Example string:
$ string="ABC_abc_123"String
Get string length:
$ echo ${#string}
Remove patterns from string
| # |
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| removes minimal matching prefixes |
| ## |
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| removes maximal matching prefixes |
| % |
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| removes minimal matching suffixes | |
| %% |
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| removes maximal matching suffixes |
| :s: |
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| n | get 'n' |
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| characters starting from position 's' (first position is 0) |
Look at these examples and you will understand the meaning of the above:
$ string="ABC_abc_123"
$ echo ${string#*_}
abc_123 123 => removes everything before first occurance of '_'
$ echo ${string##*_}
123 => removes everything before last occurance of '_'
$ echo ${string%_*}
ABC_abc abc => removes everything after last occurance of '_'
$ echo ${string%%_*}
ABC => removes everything after first occurance of '_'
$ echo ${string:s:n}
abc => gets n characters, starting from position s
cut
Cut out/print part of the content of a variable:
cut prints selected parts of a string
Options:
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string.
Most useful options:
| -c | select only these characterscharacters according to their position in the string |
| -d | delimiter=DELIM; use DELIM instead of TAB for field delimiter |
| -f | select only these fields/columns |
Examples:
$ abc="I am hungry"
$ echo $abc | cut -c 6-9
hung => returns characters 6-9
$ echo $abc | cut -c 6-
hungry => returns characters 6 to end
$ echo $abc | cut -d' ' -f 3
hungry => returns 3rd element with ' ' used as seperator
$ abc="I am hungry. Are you?"
$ echo $abc | cut -d. -f 2
Are you? => returns 2nd element with '.' used as seperator
set
When you call calling set without options but with one or more arguments, it sets the values of the command line argument variables ($1-$9$n) to its arguments.
Examples:
$ abc="I am hungry"
$ set $abc
$ echo $3
hungry
When options are specified, they set or unset shell attributes.
Options:
| -x | expands each simple command This is very useful for debugging scripts!!! | |
| +x | no expanding of commands anymore |
For more options check man page: man set
Examples:
$ abc="I am hungry"
$ set -x
$ abc="I am hungry"
+ abc='I am hungry'
& -> Send job in background
You can send a job in the background by adding a '&' at the end of the command line.
This is useful to get the prompt back - beeing being able to continue using the window - when calling a program whic which opens a nother another window, like for example Matlab, emacs, xrec.
Example:
$ matlab emacs &
In case you forgot to add the '&' you can still send an already launched job into the background with 'Ctrl-z' followed by 'bg' (for background):
Example:
$ emacs
Ctrl-z
[1]+ Stopped emacs
$ bg
[1]+ emacs &
Check and kill running processes
ps reports a snapshot of the current processesOptions
Most useful options:
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| -e | select all processes |
| -f | do full-format listing |
| -u userlist | this selects the processes whose effective user name or ID is in userlist |
Try for example:
$ ps -fu $USER
If ever you need to kill one of these jobs you can use 'kill' followed by the 'PID'.
If a normal 'kill' does not work try 'kill -9' followed by the 'PID'.