# Basic String Operations

> Source: https://learn-bash.net/basic-string-operations/
> Part of Learn Bash, free to read.

The shell allows some common string operations which can be very useful for script writing.

### String Length

```bash
#       1234567890123456
STRING="this is a string"
echo ${#STRING}            # 16
```

### Index

Find the numerical position in $STRING of any single character in $SUBSTRING that matches. Note that the ‘expr’ command is used in this case.

```bash
STRING="this is a string"
SUBSTRING="hat"
expr index "$STRING" "$SUBSTRING"     # 1 is the position of the first 't' in $STRING
```

### Substring Extraction

Extract substring of length $LEN from $STRING starting after position $POS. Note that first position is 0.

```bash
STRING="this is a string"
POS=1
LEN=3
echo ${STRING:$POS:$LEN}   # his
```

If :$LEN is omitted, extract substring from $POS to end of line

```bash
STRING="this is a string"
echo ${STRING:1}           # $STRING contents without leading character
echo ${STRING:12}          # ring
```

### Simple data extraction example:

```bash
# Code to extract the First name from the data record
DATARECORD="last=Clifford,first=Johnny Boy,state=CA"
COMMA1=`expr index "$DATARECORD" ','`  # 14 position of first comma
CHOP1FIELD=${DATARECORD:$COMMA1}       #
COMMA2=`expr index "$CHOP1FIELD" ','`
LENGTH=`expr $COMMA2 - 6 - 1`
FIRSTNAME=${CHOP1FIELD:6:$LENGTH}      # Johnny Boy
echo $FIRSTNAME
```

### Substring Replacement

```bash
STRING="to be or not to be"
```

Replace first occurrence of substring with replacement

```bash
STRING="to be or not to be"
echo ${STRING[@]/be/eat}        # to eat or not to be
```

Replace all occurrences of substring

```bash
STRING="to be or not to be"
echo ${STRING[@]//be/eat}        # to eat or not to eat
```

Delete all occurrences of substring (replace with empty string)

```bash
STRING="to be or not to be"
echo ${STRING[@]// not/}        # to be or to be
```

Replace occurrence of substring if at the beginning of $STRING

```bash
STRING="to be or not to be"
echo ${STRING[@]/#to be/eat now}    # eat now or not to be
```

Replace occurrence of substring if at the end of $STRING

```bash
STRING="to be or not to be"
echo ${STRING[@]/%be/eat}        # to be or not to eat
```

replace occurrence of substring with shell command output

```bash
STRING="to be or not to be"
echo ${STRING[@]/%be/be on $(date +%Y-%m-%d)}    # to be or not to be on 2012-06-14
```