Bash

Bash Programming

whereis bash

#!/bin/bash
echo Hello World

The first line indicates the system which program to use to run the file. Use whereis bash to see where bash is.

Redirection

&

& indicates that what follows is a file descriptor and not a file name.

File descriptor

  • stdin
  • stdout: File descriptor 1 is the standard output (stdout).
  • stderr: File descriptor 2 is the standard error (stderr).

Examples

  • ls -l > ls.txt:

    This will redirect stdout of a program (ls -l) to a file (ls.txt).

  • grep da * 2> grep-errors.txt

    This will redirect the stderr(2) of a program (grep da *) to a file (grep-errors.txt).

  • grep da * 2>&1

    This will redirect the stderr(2) of a program (grep da *) to be written to the stdout(1).

  • grep da * 2>1

    This will redirect the stderr(2) of a program (grep da *) to a file (1).

  • ls -l > ls.log 2>&1

    This will redirect stdout of a program (ls -l) to a file (ls.log) and redirect stderr(2) to stdout(1).

  • ls -l &> ls.log

    This is equivalent to ls -l > ls.log 2>&1.

  • ls -l &> ls.log &

    This is equivalent to ls -l &> ls.log, but running in a "subshell". If the current shell is closed, the "subshell" would be closed as well.

  • nohup ls -l > ls.log &

    This is equivalent to ls -l &> ls.log &, but running separately. Closing the current shell would not terminate the program.

  • nohup ls -l &

    This is equivalent to nohup ls -l > ls.log &, but the output is written into nohup.out file in the current folder.

  • grep da * > /dev/null

    This will redirect stdout of a program (grep da *) to the null device.

  • grep da * &> /dev/null

    This will redirect stdout and stderr of a program (grep da *) to the null device.

Pipes

Pipes let you use the output of a program as the input of another one.

Examples

  • ls -l | sed -e "s/[aeio]/u/g"

    Here, the following happens: first the command ls -l is executed, and it's output, instead of being printed, is sent (piped) to the sed program, which in turn, prints what it has to.

Variables

No type, no declaration.

#!/bin/bash
STR="Hello World!"
echo $STR
## Hello World!

Local variables

Local variables can be created by using the keyword local.

#!/bin/bash
#!/bin/bash
HELLO=Hello
function hello {
local HELLO=World
echo $HELLO
}
echo $HELLO
hello
echo $HELLO
## Hello
## World
## Hello

Conditionals

if..then

#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
fi

if..then...else

#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi

Conditionals with variables

#!/bin/bash
T1="foo"
T2="bar"
if [ "$T1" = "$T2" ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi

Loop

For

#!/bin/bash
for i in $( ls ); do
echo item: $i
done

While

#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done

Util

#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done

Functions

Without paramerters

#!/bin/bash
function quit {
exit
}
function hello {
echo Hello!
}
hello
quit
echo foo

With paramerters

#!/bin/bash
function quit {
exit
}
function e {
echo $1
}
e Hello
e World
quit
echo foo

User interfaces

Menus

#!/bin/bash
OPTIONS="Hello Quit"
select opt in $OPTIONS; do
if [ "$opt" = "Quit" ]; then
echo done
exit
elif [ "$opt" = "Hello" ]; then
echo Hello World
else
clear
echo bad option
fi
done

Command line

#!/bin/bash
if [ -z "$1" ]; then
echo usage: $0 directory
exit
fi
SRCD=$1
TGTD="/var/backups/"
OF=home-$(date +%Y%m%d).tgz
tar -cZf $TGTD$OF $SRCD

-z: the length of STRING is zero. Please check man test.

Return value

The return value (status) of a command is stored in the $? variable.

cmd;
echo $?;

The return value is called exit status. This value can be used to determine whether a command completed successfully or unsuccessfully. If the command exits successfully, the exit status will be zero, otherwise it will be a nonzero value.

The following script reports the success/failure status of a command:

#!/bin/bash
#Filename: success_test.sh
# Evaluate the arguments on the command line - ie success_test.sh 'ls | grep txt'
eval $@
if [ $? -eq 0 ];
then
echo "$CMD executed successfully"
else
echo "$CMD terminated unsuccessfully"
fi

References

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